Documentation: Document how to get links with git am

This adds Kees' clever apply hook to the kernel documentation so
it can be easily references when needed.

Cc: Kees Cook <[e-mail]>
Link: https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2019-July/006608.html
Signed-off-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/maintainer/configure-git.rst

···
 ::
 
 	export GPG_TTY=$(tty)
+
+
+Creating commit links to lore.kernel.org
+----------------------------------------
+
+The web site http://lore.kernel.org is meant as a grand archive of all mail
+list traffic concerning or influencing the kernel development. Storing archives
+of patches here is a recommended practice, and when a maintainer applies a
+patch to a subsystem tree, it is a good idea to provide a Link: tag with a
+reference back to the lore archive so that people that browse the commit
+history can find related discussions and rationale behind a certain change.
+The link tag will look like this:
+
+    Link: https://lore.kernel.org/r/<message-id>
+
+This can be configured to happen automatically any time you issue ``git am``
+by adding the following hook into your git:
+
+.. code-block:: none
+
+	$ git config am.messageid true
+	$ cat >.git/hooks/applypatch-msg <<'EOF'
+	#!/bin/sh
+	. git-sh-setup
+	perl -pi -e 's|^Message-Id:\s*<?([^>]+)>?$|Link: https://lore.kernel.org/r/$1|g;' "$1"
+	test -x "$GIT_DIR/hooks/commit-msg" &&
+		exec "$GIT_DIR/hooks/commit-msg" ${1+"$@"}
+	:
+	EOF
+	$ chmod a+x .git/hooks/applypatch-msg