[tz] [PATCH] mention the github repo in the main tz page

[Paul Eggert]

Thanks, I think I'll add something a bit more cautionary,
along the following lines:

An <a href="https://github.com/eggert/tz">unofficial development
repository</a> of the code and data is available
in <a href="http://git-scm.com/">Git</a> form; be careful, as this
repository is less well tested and probably contains more errors than
the official version.