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>

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