Added limitations section to outgoing webhook doc

doc/integrations/webhooks/Incoming-Webhooks.md

@@ -94,3 +94,4 @@ To see samples and community contributions, please visit <http://mattermost.org/
 
 - The `attachments` payload used in Slack is not yet supported
 - Overriding of usernames does not yet apply to notifications
+- Cannot supply `icon_emoji` to override the message icon