Clarify license

[mml]

This repo doesn't seem to explain how the sbitx code is licensed. Opening this issue is meant to spur discussion. Clarity on the license would be nice, but I don't honestly know what Farhan's wishes are (other than the intent to make the code open), nor have I carefully investigated all the other code and licenses.

The repo (and the distributed sbitx binaries) includes a lot of software by other authors. Some of it is GPL2 (e.g. mongoose, some BSD-3-clause, some MIT.

I'm not an expert, but I believe the only compatible license for the sbitx code is GPL2. GPL3 is incompatible with mongoose because it doesn't include the "any later version" clause (see the paragraph after the diagram at this link).

Thoughts?