MyFirstContribution: add "Anatomy of a Patch Series" section

MyFirstContribution: add "Anatomy of a Patch Series" section

Before describing how to send patches to the mailing list either with
GitGitGadget or 'git send-email', the MyFirstContribution tutorial
includes a small "Getting Ready to Share" section where the two
different methods are briefly introduced.

Use this section to also describe what a patch series looks like once
submitted, so that readers get an understanding of the end result before
diving into how to accomplish that end result.

Start by copying the "thread overview" section of a recent contribution
from the public-inbox web UI and explaining how each commit is a
separate mail, and point out the cover letter.

Subsequent commits will move the existing description of the purpose of
the cover letter from the 'git send-email' section to this "anatomy"
section.

Also, change the wording in the introductory paragraph to use
"contributions" instead of "patches", since this makes more sense when
talking about GitHub pull requests.

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