Documentation: convert SubmittingPatches to AsciiDoc

Documentation: convert SubmittingPatches to AsciiDoc

The SubmittingPatches document is often cited by outside parties as an
example of good practices to follow, including logical, independent
commits; patch sign-offs; and sending patches to a mailing list.
Currently, people who want to cite a particular section tend to either
refer to it by name and let the interested party search through the
document to find it, or link to a given line number on GitHub and hope
the file doesn't change.

Instead, convert the document to AsciiDoc.  Build it as part of the
technical documentation, since it is likely of interest to the same
group of people.  Provide stable links to the sections which outside
parties are likely to want to link to.  Make some minor structural
changes to organize it so that it can be formatted sanely.

Since the makefile needs a .txt extension in order to build with the
rest of the documentation, simply copy the file.  Ignore the temporary
file so it doesn't get checked in accidentally, and remove it as part of
the clean process.  Do this instead of renaming the file so that people
who have already linked to the documentation (who we're trying to help)
don't find their links broken.  Avoid symlinking since Windows will not
like that.

This allows us to render the document as part of the website for the
benefit of others who wish to link to it as well as providing a more
nicely formatted display for our community and potential contributors.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>