MyFirstContribution: add standalone section on cover letter

An explanation of the purpose of the cover letter is included in the
"Sending Patches with git send-email" / "Preparing Email" section but is
missing from the "Sending Patches via GitGitGadget" section.

Add a standalone section "The cover letter" under the "Getting Started:
Anatomy of a Patch Series" header to explain what the cover letter is
used for and to draft the cover letter of the 'psuh' topic used in the
tutorial.

For now we mostly copy content from the "Sending Patches with git
send-email" section but do not adjust that section, nor the GGG section,
to reference the new section. This is done in following commits.

Also, adjust the "Preparing Email" Asciidoc anchor to avoid conflicts.

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

Documentation/MyFirstContribution.txt

@@ -760,9 +760,52 @@ We can note a few things:
   _n_-commit series.
 - Each patch is sent as a reply to an introductory email called the _cover
   letter_ of the series, prefixed "[PATCH 0/_n_]".
-- Subsequent iterations of the patch series are labelled "[PATCH v2]", "[PATCH
-  v3]", etc. and sent with a new cover letter, itself a reply to the cover
-  letter of the previous iteration (more on that below).
+- Subsequent iterations of the patch series are labelled "PATCH v2", "PATCH
+  v3", etc. in place of "PATCH". For example, "[PATCH v2 1/3]" would be the first of
+  three patches in the second iteration. Each iteration is sent with a new cover
+  letter (like "[PATCH v2 0/3]" above), itself a reply to the cover letter of the
+  previous iteration (more on that below).
+
+NOTE: A single-patch topic is sent with "[PATCH]", "[PATCH v2]", etc. without
+_i_/_n_ numbering (in the above thread overview, no single-patch topic appears,
+though).
+
+[[cover-letter]]
+=== The cover letter
+
+In addition to an email per patch, the Git community also expects your patches
+to come with a cover letter. This is an important component of change
+submission as it explains to the community from a high level what you're trying
+to do, and why, in a way that's more apparent than just looking at your
+patches.
+
+The title of your cover letter should be something which succinctly covers the
+purpose of your entire topic branch. It's often in the imperative mood, just
+like our commit message titles. Here is how we'll title our series:
+
+---
+Add the 'psuh' command
+---
+
+The body of the cover letter is used to give additional context to reviewers.
+Be sure to explain anything your patches don't make clear on their own, but
+remember that since the cover letter is not recorded in the commit history,
+anything that might be useful to future readers of the repository's history
+should also be in your commit messages.
+
+Here's an example body for `psuh`:
+
+----
+Our internal metrics indicate widespread interest in the command
+git-psuh - that is, many users are trying to use it, but finding it is
+unavailable, using some unknown workaround instead.
+
+The following handful of patches add the psuh command and implement some
+handy features on top of it.
+
+This patchset is part of the MyFirstContribution tutorial and should not
+be merged.
+----
 
 At this point the tutorial diverges, in order to demonstrate two
 different methods of formatting your patchset and getting it reviewed.
@@ -1000,7 +1043,7 @@ but want reviewers to look at what they have so far. You can add this flag with
 Check and make sure that your patches and cover letter template exist in the
 directory you specified - you're nearly ready to send out your review!
 
-[[cover-letter]]
+[[preparing-cover-letter]]
 === Preparing Email
 
 In addition to an email per patch, the Git community also expects your patches