[PATCH 2/3] MyFirstContribution: also explain cover letter in GitGitGadget section

["Philippe Blain via GitGitGadget" <gitgitgadget@gmail.com>]

From: Philippe Blain <levraiphilippeblain@gmail.com>

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.

Mention the purpose of the cover letter in that section, and give
examples for the title and description, leveraging the excerpt extracted
from the "Sending Patches with git send-email" section in the previous
commit.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
---
 Documentation/MyFirstContribution.txt | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/Documentation/MyFirstContribution.txt b/Documentation/MyFirstContribution.txt
index 681bbefe9cd..96da32f7cef 100644
--- a/Documentation/MyFirstContribution.txt
+++ b/Documentation/MyFirstContribution.txt
@@ -808,8 +808,20 @@ 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 as the
+cover letter for your change. The cover letter describes your proposed
+contribution as a whole and is ideal to mention any context that might be
+useful for reviewers. The title of your PR should be something which
+succinctly covers the purpose of your entire topic branch, for example:
+
+----
+Adding the 'psuh' command
+----
+
+Your PR's description will used as the body of the cover letter.
+include::MyFirstContribution-coverletter.txt[]
+
+When you're happy, submit your pull request.
 
 [[run-ci-ggg]]
 === Running CI and Getting Ready to Send
-- 
gitgitgadget