README.md: add CodingGuidelines and a link for Translators

The README.md does not use the GitHub Markdown URL links [1] of
[txt](url), rather the reference method [2] of trailing empty square
brackets. The references themselves are listed at the foot of the
document. Those footers are not shown by GitHub/GitLab rendering.

Also space out the list joining instructions and clarify that
subscription is via the majordomo address.

[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

Signed-off-by: Philip Oakley <philipoakley@iee.email>

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 translation (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
-available at <https://lore.kernel.org/git/>,
+the body to majordomo@vger.kernel.org (not the Git list). The mailing
+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