Modify the GitHub Pull Request template (to reflect Git for Windows)

Git for Windows accepts pull requests; Core Git does not. Therefore we
need to adjust the template (because it only matches core Git's
project management style, not ours).

Also: direct Git for Windows enhancements to their contributions page,
space out the text for easy reading, and clarify that the mailing list
is plain text, not HTML.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,19 @@
-Thanks for taking the time to contribute to Git! Please be advised that the
-Git community does not use github.com for their contributions. Instead, we use
-a mailing list (git@vger.kernel.org) for code submissions, code reviews, and
-bug reports. Nevertheless, you can use GitGitGadget (https://gitgitgadget.github.io/)
+Thanks for taking the time to contribute to Git!
+
+Those seeking to contribute to the Git for Windows fork should see
+http://gitforwindows.org/#contribute on how to contribute Windows specific
+enhancements.
+
+If your contribution is for the core Git functions and documentation
+please be aware that the Git community does not use the github.com issues
+or pull request mechanism for their contributions.
+
+Instead, we use the Git mailing list (git@vger.kernel.org) for code and
+documentation submissions, code reviews, and bug reports. The
+mailing list is plain text only (anything with HTML is sent directly
+to the spam folder).
+
+Nevertheless, you can use GitGitGadget (https://gitgitgadget.github.io/)
 to conveniently send your Pull Requests commits to our mailing list.
 
 For a single-commit pull request, please *leave the pull request description