Adds System Log documentation

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+# System Log
+
+## Viewing the System Log from LuCi
+
+Select the `Status >> System Log` menu entry.
+
+![alt text](images/librecmc-selecting-system-log.png "Selecting the
+ System Log page in LuCi")
+
+![alt text](images/librecmc-system-log.png "The System Log page in
+ LuCi")
+
+The `System Log` page in LuCi does not have an interface for filtering
+output. Therefore, you will likely want to use the shell interface.
+
+## Viewing the System Log from the shell
+
+Once logged in via SSH, use the `logread` command
+
+```
+Usage: logread [options]
+Options:
+    -s <path>          Path to ubus socket
+    -l <count>         Got only the last 'count' messages
+    -e <pattern>       Filter messages with a regexp
+    -r <server> <port> Stream message to a server
+    -F <file>          Log file
+    -S <bytes>         Log size
+    -p <file>          PID file
+    -h <hostname>      Add hostname to the message
+    -P <prefix>        Prefix custom text to streamed messages
+    -f                 Follow log messages
+    -u                 Use UDP as the protocol
+    -t                 Add an extra timestamp
+    -0                 Use \0 instead of \n as trailer when using TCP
+```
+
+For example:
+
+```
+root@libreCMC:~# logread | grep 'kern\.warn'
+Mon Jan 15 20:22:01 2018 kern.warn kernel: [    0.000000] No valid device tree found, continuing without
+Mon Jan 15 20:22:01 2018 kern.warn kernel: [    0.000000] Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
+Mon Jan 15 20:22:01 2018 kern.warn kernel: [    0.000000] Primary data cache 32kB, 4-way, VIPT, cache aliases, linesize 32 bytes
+Mon Jan 15 20:22:01 2018 kern.warn kernel: [    0.669305] Crashlog allocated RAM at address 0x3f00000
+Mon Jan 15 20:22:01 2018 kern.warn kernel: [    0.752178] m25p80 spi0.0: found mx25l12805d, expected m25p80
+```
+
+One should be able to use `logread -e` instead of `grep`, but it seems
+that not all the same regular expressions work for both:
+
+```
+root@libreCMC:~# logread -e 'kern\.warn' # and other similar variations
+(no output)
+````
+
+The system log is contained in a limited size, circular buffer in
+memory. So, if you have some process writing messages periodically,
+this will eventually erase messages that were only written once.
+
+## Configuring the system log
+
+TODO
+
+## Monitoring the system log
+
+TODO

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