Add schema for buttonmap.xml

Schema.md

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+# Schema for button map
+
+## buttonmap.xml
+
+A button map has the following properties:
+
+1. Button map schema version
+2. List of controllers
+
+A controller has the following properties:
+
+1. Controller ID (equal to Kodi add-on ID)
+2. Libretro device type (from the libretro API)
+3. Libretro subclass index (defined by the core)
+4. List of features
+
+The purpose of the properties is to assign the libretro device type and subclass to the specified add-on ID.
+
+A `<feature>` tag maps a feature name to a libretro constant. It has the following properties:
+
+1. Feature name (from the kodi-game-controllers repo)
+2. Libretro constant (from the libretro API)