Add license

[legoandmars]

I'd love to fork a Firefox port of this extension, but I'm technically not legally allowed to modify or redistribute this code since there's no open source license attached.

[PossiblePanda]

I mean it's a public GitHub repository, I'd assume you wouldn't have an issue with forking this repo

[Lightning11wins]

I think they're just going with "better safe than sorry". A license would be nice to avoid future confusion.

[Luew2]

@anatolyzenkov please add a license!

I would love to use some of your css grabbing -- but don't want to take without explicit permission. Without a license we can't use your code :(