community: link to SourceHut's plain text email guide
community: link to SourceHut's plain text email guide

SourceHut maintains a nice guide at https://useplaintext.email/
explaining how to set up different email client to send plain text
email.

Let's add a link to it when we mention that the Git mailing list only
accepts plain text, to help our readers.

git/git-scm.com,git/git-scm.com,git/git-scm.com,git/git-scm.com