MyFirstContribution: reference "The cover letter" in GitGitGadget sec…

…tion

The "Sending Patches via GitGitGadget" section mentions that the PR
title and description will be used as the cover letter, but does not
explain what is a cover letter or what should be included in it.

Refer readers to the new "The cover letter" section added in a previous
commit.

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

Documentation/MyFirstContribution.txt

@@ -899,8 +899,12 @@ https://github.com/gitgitgadget/git and open a PR either with the "New pull
 request" button or the convenient "Compare & pull request" button that may
 appear with the name of your newly pushed branch.
 
-Review the PR's title and description, as it's used by GitGitGadget as the cover
-letter for your change. When you're happy, submit your pull request.
+Review the PR's title and description, as they're used by GitGitGadget
+respectively as the subject and body of the cover letter for your change. Refer
+to <<cover-letter,"The cover letter">> above for advice on how to title your
+submission and what content to include in the description.
+
+When you're happy, submit your pull request.
 
 [[run-ci-ggg]]
 === Running CI and Getting Ready to Send