add instructions on how to send patches to the mailing list with Gmail

Gmail is one of the most popular email providers in the world. Now that Gmail
supports IMAP, sending properly formatted patches via `git imap-send` is
trivial. This section in SubmittingPatches explains how to do so.

Signed-off-by: Tom Preston-Werner <tom@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -456,3 +456,30 @@ This should help you to submit patches inline using KMail.
 
 5) Back in the compose window: add whatever other text you wish to the
 message, complete the addressing and subject fields, and press send.
+
+
+Gmail
+-----
+
+Submitting properly formatted patches via Gmail is simple now that
+IMAP support is available. First, edit your ~/.gitconfig to specify your
+account settings:
+
+[imap]
+	folder = "[Gmail]/Drafts"
+	host = imaps://imap.gmail.com
+	user = user@gmail.com
+	pass = p4ssw0rd
+	port = 993
+	sslverify = false
+
+Next, ensure that your Gmail settings are correct. In "Settings" the
+"Use Unicode (UTF-8) encoding for outgoing messages" should be checked.
+
+Once your commits are ready to send to the mailing list, run the following
+command to send the patch emails to your Gmail Drafts folder.
+
+	$ git format-patch -M --stdout origin/master | git imap-send
+
+Go to your Gmail account, open the Drafts folder, find the patch email, fill
+in the To: and CC: fields and send away!