Add license

[nacho]

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Fixes https://github.com/hexchat/gtk-win32/issues/31

See that I used GPL 2+ since it is the one used in hexchat and it is just a proposal. Whether you want [A|L]GPL [2|3]/+ it is up to you and the authors. Thanks

[Arnavion]

Since this repo contains derivative files from a bunch of other software, every dependency's directory will probably need a copy of corresponding upstream license file (in case the project files are derived from upstream) or third-party license file (in case the project files are derived from some other place, like harfbuzz and gettext-runtime), which will clarify license of that particular dependency + copyrights.

If that is too much effort (and I think it is), at the very least there should be an assertion somewhere (eg in the README) that our license + copyright only applies to build.ps1, etc. and the individual dependencies' files are under their own license + copyright.