MyFirstContribution: drop PR description for GGG single-patch contributions

MyFirstContribution: drop PR description for GGG single-patch contributions

By default, GitHub prefills the PR description using the commit message
for single-commit PRs. This results in a duplicate commit message below
the three-dash line if the contributor does not empty out the PR
description before submitting, which adds noise for reviewers.

Add a note to that effect in MyFirstContribution.txt.

This partly addresses:
https://github.com/gitgitgadget/gitgitgadget/issues/340

Signed-off-by: Philippe Blain <redacted>
Signed-off-by: Junio C Hamano <redacted>