.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>

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