Add gmail instructions for submission.torproject.org

Hi! Please merge this only if you think it will be helpful, and feel free to clean it up or edit it any way you want.

howto/submission.md

@@ -89,6 +89,43 @@ password. You should *NOT* get a certificate warning, a real cert
 
 `TLS` connection security, on port `465` is supported and encouraged.
 
+## Gmail configuration
+
+**NOTE**: This section explains how to reconfigure a gmail account that
+is _already_ configured to use a torproject.org address.  If you need to add
+a new address from scratch, the process will be a little different.
+
+1. Click  on "Settings".  (That  should be the  big gear icon  towards thetop
+right of your window.)
+
+2. A "quick settings" menu should open.  Click on the "See all settings"
+button at the top of that menu.
+
+3. This will take you to a "Settings" page. Click on the "Accounts and
+Import" tab at the top of the page.
+
+4. Under "Send mail as", find the address for <yourname@torproject.org>, and
+lick the "edit info" link to the right of that account. (If the address isn't
+there, then your gmail account isn't set up to use your TPO account. You'll
+need to click "Add another email address" instead, and the process below will
+be slightly different.)
+
+5. A new "Edit email address" popup should open.  Click "Next step" on it.
+
+6. Finally, you'll be at a window that says "Edit email address".  Fill it
+out like this:
+
+ - Select "Send through torproject.org SMTP servers".
+ - Set "SMTP Server:" to "submission.torproject.org"
+ - Set "Port:" to 587.
+ - Set "Username:" to your username (_without_ "@torproject.org").
+ - Set "Password:" to the email submission password that you configured.
+ - Select "Use TLS", not "Use SSL".
+
+Double-check everything, then click "Save Changes".  Gmail will try
+authenticating to the SMTP server; if it's successful, then the popup window
+will close and your account will be updated.
+
 # How-to
 
 <!-- more in-depth procedure that may require interpretation -->