Add a README paragraph about data licensing.

README.md

@@ -15,3 +15,5 @@ The key scripts in this SDK are:
 To launch your project from the development environment you must first compile the project by running `make.cmd` (Windows), or opening a terminal in the SDK directory and running `make` (Linux / macOS).  You can then run `launch-game.cmd` (Windows) or `launch-game.sh` (Linux / macOS) to run your game.
 
 The `example` mod included in this repository provides the bare minimum structure to launch to the in-game main menu for the sole purpose of demonstrating the SDK.  See [Getting Started](https://github.com/OpenRA/OpenRAModTemplate/wiki/Getting-Started) on the Wiki for instructions on how to adapt this template for your own projects.  For common questions, please see the [FAQ](https://github.com/OpenRA/OpenRAModSDK/wiki/FAQ).  See [Updating to a new SDK or Engine version](https://github.com/OpenRA/OpenRAModSDK/wiki/Updating-to-a-new-SDK-or-Engine-version) for a guide on updating your mod a newer OpenRA release.
+
+The OpenRA engine and SDK scripts are made available under the [GPLv3](https://github.com/OpenRA/OpenRA/blob/bleed/COPYING) license, and any executable code developed by a mod and loaded by the engine (i.e. custom mod DLLs, lua scripts) must be released under a compatible license.  Your mod data files (artwork, sound files, yaml, etc) are not part of your mod's source code, so your are free to distribute these assets under different terms (e.g. allowing redistribution in unmodified form, but not for use in other works).