.github/CONTRIBUTING.md: link to SubmittingPatches on git-scm.com
The relative link to SubmittingPatches is broken when viewed through
GitHub's specialized "Contributing" tab. Update the link to point to
the documentation on git-scm.com to be consistent with other links in
the same file. Also, wrap the line to improve readability.
Signed-off-by: Abdalrhman Mohamed <Eng.Abdalrhman.Abdalmonem@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>