.github/PULL_REQUEST_TEMPLATE.md: add a note about single-commit PRs

Contributors using Gitgitgadget continue to send single-commit PRs with
their commit message text duplicated below the three-dash line,
increasing the signal-to-noise ratio for reviewers.

This is because Gitgitgadget copies the pull request description as an
in-patch commentary, for single-commit PRs, and _GitHub_ defaults to
prefilling the pull request description with the commit message, for
single-commit PRs (followed by the content of the pull request
template).

Add a note in the pull request template mentioning that for
single-commit PRs, the PR description should thus be kept empty, in the
hope that contributors read it and act on it.

This partly addresses:
gitgitgadget/gitgitgadget#340

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phil-blain

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,4 +4,7 @@ a mailing list (git@vger.kernel.org) for code submissions, code reviews, and
 bug reports. 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
+empty*: your commit message itself should describe your changes.
+
 Please read the "guidelines for contributing" linked above!