Update CONTRIBUTING.md

added details about the license and the supported distributions.

CONTRIBUTING.md

 _This document will be expanded upon._
 
 You'll want to set up a [local development environment](https://github.com/sovereign/sovereign/wiki/Development-Environment) so that you don’t have to test on a remote server.
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+Make sure you agree with the license (GPLv3). See [LICENSE.md](./LICENSE.md) for details.
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+If you issue a pull request, please specify what distribution you used for testing (if any).
+Code that is committed to the master branch should work with both Debian 7 and Ubuntu 14.04 LTS (Debian 8 support is coming up).