Explain extending CMD.

Author: saurtron

rts/Lua/LuaConstCMD.cpp

@@ -18,6 +18,22 @@
  *   master list of command IDs.
  * - Also supports integer keys, and those perform reverse mapping of command IDs.
  *
+ * The table can be extended by games through lua, in order to define custom
+ * commands.
+ *
+ * To extend, do:
+ *
+ * ```LUA
+ * local MY_CUSTOM_COMMAND = 57600 -- avoid conflicts
+ * CMD['MY_CUSTOM_COMMAND'] = MY_CUSTOM_COMMAND
+ * CMD[MY_CUSTOM_COMMAND] = 'MY_CUSTOM_COMMAND'
+ * ```
+ *
+ * For now there is no mechanism to avoid id conflicts, so you will have
+ * to be careful. Also you should put this code in a place where both
+ * Synced, Unsynced, and all lua environments can import it, otherwise
+ * it won't be available everywhere.
+ *
  * @see Spring.GiveOrderToUnit
  * @see Spring.GiveOrderToUnitArray
  * @enum CMD