Documentation/process: expand plain-text advice

This adds a link to https://useplaintext.email to email-clients.rst,
which is a more exhaustive resource on configuring various mail clients
for plain text use. submitting-patches.rst is also updated to direct
readers to email-clients.rst to equip new contributors with the
requisite knowledge to become a good participant on the mailing lists.

Signed-off-by: Drew DeVault <sir@cmpwn.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20200903160545.83185-3-sir@cmpwn.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/email-clients.rst

@@ -25,6 +25,11 @@ attachments, but then the attachments should have content-type
 it makes quoting portions of the patch more difficult in the patch
 review process.
 
+It's also strongly recommended that you use plain text in your email body,
+for patches and other emails alike. https://useplaintext.email may be useful
+for information on how to configure your preferred email client, as well as
+listing recommended email clients should you not already have a preference.
+
 Email clients that are used for Linux kernel patches should send the
 patch text untouched.  For example, they should not modify or delete tabs
 or spaces, even at the beginning or end of lines.

Documentation/process/submitting-patches.rst

@@ -396,6 +396,9 @@ for their time.  Code review is a tiring and time-consuming process, and
 reviewers sometimes get grumpy.  Even in that case, though, respond
 politely and address the problems they have pointed out.
 
+See :ref:`Documentation/process/email-clients.rst` for recommendations on email
+clients and mailing list etiquette.
+
 
 Don't get discouraged - or impatient
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