Update README to indicate Mattermost compatibility.

README.md

@@ -1,4 +1,4 @@
-Slack Plugin for Graylog
+Slack/Mattermost Plugin for Graylog
 ========================
 
 [![Build Status](https://travis-ci.org/Graylog2/graylog-plugin-slack.svg)](https://travis-ci.org/Graylog2/graylog-plugin-slack)
@@ -11,7 +11,7 @@ Please use version 2.1.0 of this plugin if you are still running Graylog 1.x
 
 ![](https://github.com/Graylog2/graylog-plugin-slack/blob/master/screenshot.png)
 
-This plugin can notify [Slack](https://www.slack.com) channels about triggered alerts in Graylog (Alarm Callback) and also forward each message routed into a stream (Message Output) in realtime.
+This plugin can notify [Slack](https://www.slack.com) or [Mattermost](http://www.mattermost.org) channels about triggered alerts in Graylog (Alarm Callback) and also forward each message routed into a stream (Message Output) in realtime.
 
 #### Short mode message output:
 
@@ -42,10 +42,18 @@ Restart `graylog-server` and you are done.
 
 ## Usage
 
+### For Slack:
+
 #### Step 1: Create Slack Incoming Webhook
 
 Create a new Slack Incoming Webhook (https://<organization>.slack.com/services/new/incoming-webhook) and copy the URL it will present to you. It will ask you to select a Slack channel but you can override it in the plugin configuration later.
 
+### For Mattermost:
+
+#### Step 1: Create Mattermost Incoming Webhook
+
+Enable Webhooks in general and create an incoming Webhook for Graylog as described in the [Mattermost docs](http://docs.mattermost.com/developer/webhooks-incoming.html).
+
 #### Step 2: Create alarm callback or message output
 
 Create a "Slack alarm callback" on the "Manage alerts" page of your stream. Enter the requested configuration (use the Incoming Webhook URL you created in step 1) and save. Make sure you also configured alert conditions for the stream so that the alerts are actually triggered.