Add an explicit license

[benjaminbauer]

In order to future proof this plugin it is necessary to put it under a license that allows forking in case this repository gets abandoned. See #12 (comment)

[alexcochran]

@akaalias are you alright with tossing a license on here? I was trying to get one added to a downstream project and someone pointed out we need to start here. I'm happy to make a quick PR for one if you're short on time.