.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: https://github.com/gitgitgadget/gitgitgadget/issues/340 Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>