heads up: please use USE_GITHUB=yes when packaging software hosted on GH

[Tobias Nygren <tnn%NetBSD.org@localhost>]

Hi,

I added a helper mk fragment to contain the github distinfo mess,
inspired by FreeBSD. Please refer to these instructions:

https://www.netbsd.org/docs/pkgsrc/fixes.html#build.fetch.github

When converting packages to USE_GITHUB, please remember to
remove MASTER_SITES, PKGNAME, DIST_SUBDIR and WRKSRC overrides
as those are no longer required in the typical case.

Thank you,
-Tobias