[PATCH] docs: document X-Kallithea-Notification-Type header

[Thomas De Schampheleire]

# HG changeset patch
# User Thomas De Schampheleire <thomas.de.schampheleire at gmail.com>
# Date 1478796845 -3600
#      Thu Nov 10 17:54:05 2016 +0100
# Node ID d26d51fa51764ee38799006e98b6a5b0bcb8bf2b
# Parent  84099c5e092034542419fd5792f1e9b02110e520
docs: document X-Kallithea-Notification-Type header

diff --git a/docs/usage/email.rst b/docs/usage/email.rst
--- a/docs/usage/email.rst
+++ b/docs/usage/email.rst
@@ -57,6 +57,18 @@ configuration setting disregarded).
 The subject of these emails can optionally be prefixed with the value of
 ``email_prefix`` in the configuration file.
 
+A Kallithea-specific header indicating the email type will be added to each
+email. This header can be used for email filtering. The header is of the form:
+
+    X-Kallithea-Notification-Type: <type>
+
+where ``<type>`` is one of:
+
+- ``pull_request``: you are invited as reviewer in a pull request
+- ``pull_request_comment``: a comment was given on a pull request
+- ``cs_comment``: a comment was given on a changeset
+- ``registration``: a new user was registered
+- ``message``: another type of email
 
 Error emails
 ------------