README.md: add CodingGuidelines and a link for Translators

Before being told how to submit patches, new contributors need
to be told how to code for, or how to contribute translation to,
the project.  Add references to the CodingGuidelines and the
README document on localization.

Also, split out the instructions to join the list and clarify
that subscription is via the majordomo address.

We use GitHub Markdown reference [2,3] with trailing empty square
brackets, to match existing text in the file.  On GitHub/GitLab
pages, the footer references matching the empty [] are not shown
on the web page. We could switch to using [text](url) form [1]
if we wanted to, but that is not done as part of this patch.

[1] https://docs.github.com/en/github/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#links
[2] https://gist.github.com/ChrisTollefson/a3af6d902a74a0afd1c2d79aadc9bb3f#reference-links
[3] https://github.github.com/gfm/#example-561 (and the para aboveit)

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

README.md

@@ -32,10 +32,16 @@ installed).
 The user discussion and development of Git take place on the Git
 mailing list -- everyone is welcome to post bug reports, feature
 requests, comments and patches to git@vger.kernel.org (read
-[Documentation/SubmittingPatches][] for instructions on patch submission).
+[Documentation/SubmittingPatches][] for instructions on patch submission
+and [Documentation/CodingGuidelines][]).
+
+Those wishing to help with error message, usage and informational message
+string translations (localization l10) should see [po/README.md][]
+(a `po` file is a Portable Object file that holds the translations).
+
 To subscribe to the list, send an email with just "subscribe git" in
-the body to majordomo@vger.kernel.org. The mailing list archives are
+the body to majordomo@vger.kernel.org (not the Git list). The mailing
-available at <https://lore.kernel.org/git/>,
+list archives are available at <https://lore.kernel.org/git/>,
 <http://marc.info/?l=git> and other archival sites.
 
 Issues which are security relevant should be disclosed privately to
@@ -64,3 +70,5 @@ and the name as (depending on your mood):
 [Documentation/giteveryday.txt]: Documentation/giteveryday.txt
 [Documentation/gitcvs-migration.txt]: Documentation/gitcvs-migration.txt
 [Documentation/SubmittingPatches]: Documentation/SubmittingPatches
+[Documentation/CodingGuidelines]: Documentation/CodingGuidelines
+[po/README.md]: po/README.md