Add plugin quickstart documentation

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+# Plugins & Themes
+
+## Concepts
+
+Themes are exactly the same than plugins, except that:
+ * Their name starts with `peertube-theme-` instead of `peertube-plugin-`
+ * They cannot declare server code (so they cannot register server hooks or settings)
+ * CSS files are loaded by client only if the theme is chosen by the administrator or the user
+
+### Hooks
+
+A plugin registers functions in JavaScript to execute when PeerTube (server and client) fires events. There are 3 types of hooks:
+ * `filter`: used to filter functions parameters or return values. 
+ For example to replace words in video comments, or change the videos list behaviour
+ * `action`: used to do something after a certain trigger. For example to send a hook every time a video is published
+ * `static`: same than `action` but PeerTube waits their execution
+ 
+Example:
+
+```js
+registerHook({
+  target: 'action:application.listening',
+  handler: () => displayHelloWorld()
+})
+```
+
+On server side, these hooks are registered by the `library` file defined in `package.json`.
+
+```json
+{
+  ...,
+  "library": "./main.js",
+  ...,
+}
+```
+
+
+On client side, these hooks are registered by the `clientScripts` files defined in `package.json`.
+All client scripts have scopes so PeerTube client only loads scripts it needs:
+
+```json
+{
+  ...,
+  "clientScripts": [
+    {
+      "script": "client/common-client-plugin.js",
+      "scopes": [ "common" ]
+    },
+    {
+      "script": "client/video-watch-client-plugin.js",
+      "scopes": [ "video-watch" ]
+    }
+  ],
+  ...
+}
+```
+
+### Static files
+
+Plugins can declare static directories that PeerTube will serve (images for example) 
+from `/plugins/{plugin-name}/{plugin-version}/static/` 
+or `/themes/{theme-name}/{theme-version}/static/` routes.
+
+### CSS
+
+Plugins can declare CSS files that PeerTube will automatically inject in the client.
+
+### Server helpers
+
+**Only for plugins**
+
+#### Settings
+
+Plugins can register settings, that PeerTube will inject in the administration interface.
+
+Example:
+
+```js
+registerSetting({
+  name: 'admin-name',
+  label: 'Admin name',
+  type: 'input',
+  default: 'my super name'
+})
+
+const adminName = await settingsManager.getSetting('admin-name')
+```
+
+##### Storage
+
+Plugins can store/load JSON data, that PeerTube will store in its database (so don't put files in there).
+
+Example:
+
+```js
+const value = await storageManager.getData('mykey')
+await storageManager.storeData('mykey', 'myvalue')
+```
+
+### Publishing
+
+PeerTube plugins and themes should be published on [NPM](https://www.npmjs.com/) so that PeerTube indexes
+take into account your plugin (after ~ 1 day). An official PeerTube index is available on https://packages.joinpeertube.org/ (it's just a REST API, so don't expect a beautiful website).
+
+## Write a plugin/theme
+
+Steps:
+ * Find a name for your plugin or your theme (must not have spaces, it can only contain lowercase letters and `-`)
+ * Add the appropriate prefix:
+   * If you develop a plugin, add `peertube-plugin-` prefix to your plugin name (for example: `peertube-plugin-mysupername`)
+   * If you develop a theme, add `peertube-theme-` prefix to your theme name (for example: `peertube-theme-mysupertheme`)
+ * Clone the quickstart repository
+ * Configure your repository
+ * Update `README.md`
+ * Update `package.json`
+ * Register hooks, add CSS and static files
+ * Test your plugin/theme with a local PeerTube installation
+ * Publish your plugin/theme on NPM
+
+### Clone the quickstart repository
+
+If you develop a plugin, clone the `peertube-plugin-quickstart` repository:
+
+```
+$ git clone https://framagit.org/framasoft/peertube/peertube-plugin-quickstart.git peertube-plugin-mysupername
+```
+
+If you develop a theme, clone the `peertube-theme-quickstart` repository:
+
+```
+$ git clone https://framagit.org/framasoft/peertube/peertube-theme-quickstart.git peertube-theme-mysupername
+```
+
+### Configure your repository
+
+Set your repository URL:
+
+```
+$ cd peertube-plugin-mysupername # or cd peertube-theme-mysupername
+$ git remote set-url origin https://your-git-repo
+```
+
+### Update README
+
+Update `README.md` file:
+
+```
+$ $EDITOR README.md
+```
+
+### Update package.json
+
+Update the `package.json` fields:
+   * `name` (should start with `peertube-plugin-` or `peertube-theme-`)
+   * `description`
+   * `homepage`
+   * `author`
+   * `bugs`
+   * `engine.peertube` (the PeerTube version compatibility, must be `>=x.y.z` and nothing else)
+   
+**Caution:** Don't update or remove other keys, or PeerTube will not be able to index/install your plugin.
+If you don't need static directories, use an empty `object`: 
+
+```json
+{
+  ...,
+  "staticDirs": {},
+  ...
+}
+```
+
+And if you don't need CSS files, use an empty `array`:
+
+```json
+{
+  ...,
+  "css": [],
+  ...
+}
+```
+
+### Write code
+
+Now you can register hooks or settings, write CSS and add static directories to your plugin or your theme :)
+
+**Caution:** It's up to you to check the code you write will be compatible with the PeerTube NodeJS version, 
+and will be supported by web browsers.
+If you want to write modern JavaScript, please use a transpiler like [Babel](https://babeljs.io/).
+
+### Test your plugin/theme
+
+You'll need to have a local PeerTube instance:
+ * Follow the [dev prerequisites](https://github.com/Chocobozzz/PeerTube/blob/develop/.github/CONTRIBUTING.md#prerequisites) 
+ (to clone the repository, install dependencies and prepare the database)
+ * Build PeerTube (`--light` to only build the english language): 
+
+```
+$ npm run build -- --light
+```
+ 
+ * Run it  (you can access to your instance on http://localhost:9000): 
+
+```
+$ NODE_ENV=test npm start
+```
+
+ * Register the instance via the CLI: 
+
+```
+$ node ./dist/server/tools/peertube.js auth add -u 'http://localhost:9000' -U 'root' --password 'test'
+```
+
+Then, you can install or reinstall your local plugin/theme by running:
+
+```
+$ node ./dist/server/tools/peertube.js plugins install --path /your/absolute/plugin-or-theme/path
+```
+
+### Publish
+
+Go in your plugin/theme directory, and run:
+
+```
+$ npm publish
+```
+
+Every time you want to publish another version of your plugin/theme, just update the `version` key from the `package.json`
+and republish it on NPM. Remember that the PeerTube index will take into account your new plugin/theme version after ~24 hours.
+