Documented API commands.

README.md

@@ -145,3 +145,28 @@ will play. On December 2nd, track 2 followed by track 1. On
 December 3rd, tracks 3, 1 and 2. From December 24th on, track
 24 followed by tracks 1-23. So your kid will get the "daily track"
 first, followed by all previous tags in the right order.
+
+#### API
+You can send commands to ESMP3 using three different ways:
+* Through a websocket connection to `ws://<IP>/ws`.
+* Through the serial console using an USB cable.
+* Via HTTP POST request to `http://<IP>/cmd`, having the
+  command in the variable `cmd`.
+
+Supported commands are:
+| Command | Action |
+|---------|--------|
+| `play <PATH>` | Starts playing the given path. Path may be a path on the
+sd card or a http(s) URL of a webstream (direct links to mp3/4/ogg streams,
+PLS files, M3U files or podcast XML feeds are supported). |
+| `play` | Continues playing the previously played thing. |
+| `stop` | Stops playing. |
+| `volume=<X>` | Sets the volume to X (0-255). |
+| `track_prev` | Starts the previous track, if available. |
+| `track_next` | Starts the next track, if available. |
+| `track=<X>` | Starts playing track no. X of the currently playing album. |
+| `reset_vs1053` | Resets the VS1053 audio chip. |
+| `reboot` | Reboots ESMP3. |
+| `add_mapping=<ID>=<PATH>` | Adds a mapping between RFID card <ID> and path
+<PATH>. See `play` for valid path formats. |
+| `update` | Runs an update check. |