Create drivers README.md

drivers/README.md

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+# Mattermost
+
+This is a quick and dirty driver to send messages to a [Mattermost](https://mattermost.com/) server via an [incoming webhook](https://developers.mattermost.com/integrate/webhooks/incoming/). 
+
+## Requirements
+
+* Mattermost Incoming Webhook URL
+
+## Installation
+
+* Drivers Code -> New Driver -> Import URL: https://github.com/jglemza/hubitat/raw/main/drivers/mattermost.groovy
+* Go to Devices section and + Add Device -> Virtual
+* Enter a Device Name (e.g., Mattermost hubitat channel)
+* Set the Type to Mattermost
+* Save Device
+* Edit your new device and set its preferences (i.e., Webhook URL)