Add CONTRIBUTING file

To better manage copyright assignment (and reduce back & forths on pull requests as seen in https://github.com/ValveSoftware/source-sdk-2013/commit/823818305dd9e4712c9bea17e80ead3c1b52fd3f#commitcomment-3520133) put a statement in CONTRIBUTING about the requirement, a common repo file to specify how to contribute.
IANAL, but this should do.