README.md: Add a bit of documentation

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

README.md

@@ -1,5 +1,42 @@
 # Kaleidoscope-LEDEffect-Rainbow
 
-This is a plugin for [Kaleidoscope][fw], adding some Rainbow LED effects.
+![status][st:stable] [![Build Status][travis:image]][travis:status]
 
- [fw]: https://github.com/keyboardio/Kaleidoscope
+ [travis:image]: https://travis-ci.org/keyboardio/Kaleidoscope-LEDEffect-Rainbow.svg?branch=master
+ [travis:status]: https://travis-ci.org/keyboardio/Kaleidoscope-LEDEffect-Rainbow
+
+ [st:stable]: https://img.shields.io/badge/stable-✔-black.svg?style=flat&colorA=44cc11&colorB=494e52
+ [st:broken]: https://img.shields.io/badge/broken-X-black.svg?style=flat&colorA=e05d44&colorB=494e52
+ [st:experimental]: https://img.shields.io/badge/experimental----black.svg?style=flat&colorA=dfb317&colorB=494e52
+
+Two colorful rainbow effects are implemented by this plugin: one where the
+rainbow waves through the keys, and another where the LEDs breathe though the
+colors of a rainbow. The difference is that in the first case, we have all the
+rainbow colors on display, and it waves through the keyboard. In the second
+case, we have only one color at a time, for the whole board, and the color
+cycles through the rainbow's palette.
+
+## Using the extension
+
+To use the plugin, include the header, and tell the firmware to use either (or
+both!) of the effects:
+
+```c++
+#include <Kaleidoscope-LEDEffect-Rainbow.h>
+
+void setup() {
+  Kaleidoscope.use(&LEDRainbowEffect, &LEDRainbowWaveEffect);
+
+  Kaleidoscope.setup();
+}
+```
+
+## Plugin methods
+
+The plugin provides two objects: `LEDRainbowEffect`, and `LEDRainbowWaveEffect`,
+neither of which have any public methods or properties, outside of those
+provided by all LED modes.
+
+## Dependencies
+
+* [Kaleidoscope-LEDControl](https://github.com/keyboardio/Kaleidoscope-LEDControl)