doc/hgrc-email: Example config for using 'hg email' to contribute

The initial setup can be tricky if you don't know what and were settings
should be added.

This should maybe also go into site/doc/contributing

doc/hgrc-email.ini

@@ -0,0 +1,16 @@
+# Settings for using `hg email`
+
+[email]
+# See `hg help config.email`
+to = prosody-dev@googlegroups.com
+
+[smtp]
+# See `hg help config.smtp`
+host = mail.example.com
+port = submission
+tls = starttls
+username =
+
+[extensions]
+# command to send changesets as (a series of) patch emails
+patchbomb =