Add dual-licensing notice to Forge mapping code

BSD-licensed version here: https://gist.github.com/riking/2f330f831c30e2276df7

As a note, @libraryaddict is not bound by the BSD license, but rather by the implicit license granted from me sending a pull request. Therefore, a LICENSE file is not needed in the distributed jar.
Merge pull request #38 from riking/patch-2

Add dual-licensing notice to Forge mapping code