added mattermost documentation

docs/services/mattermost.md

@@ -0,0 +1,63 @@
+# MatterMost
+
+## URL format
+
+## Creating a Webhook in MatterMost
+
+1. Open up the Integrations page by clicking on *Integrations* within the menu
+![Screenshot 1](mattermost/1.PNG)
+
+2. Click *Incoming Webhooks*
+![Screenshot 2](mattermost/2.PNG)
+
+3. Click *Add Incoming Webhook*
+![Screenshot 3](mattermost/3.PNG)
+
+4. Fill in the information for the webhook and click *Save*
+![Screenshot 4](mattermost/4.PNG)
+
+5. If you did everything correctly, MatterMost will give you the *URL* to your newly created webhook
+![Screenshot 5](mattermost/5.PNG)
+
+6. Format the service URL
+```
+https://your-domain.com/hooks/bywsw8zt5jgpte3nm65qjiru6h
+                              └────────────────────────┘
+                                        token
+mattermost://your-domain.com/bywsw8zt5jgpte3nm65qjiru6h
+                             └────────────────────────┘
+                                       token
+```
+
+## Additional URL configuration
+
+Mattermost provides functionality to post as another user or to another channel as configured in the webhook configuration.
+<br/>
+To do this, you can add a *user* and/or *channel* to the service URL.
+
+```
+mattermost://your-domain.com/bywsw8zt5jgpte3nm65qjiru6h/shoutrrrUser/shoutrrrChannel
+                             └────────────────────────┘ └──────────┘ └─────────────┘
+                                       token                user        channel
+```
+
+Be aware that if you want to pass user or channel only, you have to leave the other one blank.
+
+## Passing parameters via code
+
+If you want to, you also have teh possibility to pass parameters to the `send` function.
+<br/>
+The following example contains all parameters, that are currently supported.
+
+```gotemplate
+params := (*types.Params)(
+	&map[string]string{
+		"username": "overwriteUserName",
+		"channel": "overwriteChannel",
+	},
+)
+
+service.Send("this is a message", params)
+```
+
+This will overwrite any options, that you passed via URL.