Documentation: add section about git to email-clients.txt

Email clients info for Linux

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Git

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These days most developers use `git send-email` instead of regular

email clients.  The man page for this is quite good.  On the receiving

end, maintainers use `git am` to apply the patches.

If you are new to git then send your first patch to yourself.  Save it

as raw text including all the headers.  Run `git am raw_email.txt` and

then review the changelog with `git log`.  When that works then send

the patch to the appropriate mailing list(s).

General Preferences

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Patches for the Linux kernel are submitted via email, preferably as