release Twinkle under CC BY-SA 3.0

[NovemLinguae]

Fix #1690
Fix #252

Add a LICENSE.md file that is identical to the license used on Wikipedia, CC BY-SA 3.0

I'll avoid self merging this one. I'm not an expert on copyright. So someone else should read the two linked threads, decide if this is OK, and then make a decision on merging.

[NovemLinguae]

@atlight, @legoktm, @siddharthvp, are any of you guys good at code licensing? Got time to provide an opinion on this patch? Thanks.

[legoktm]

I think @atlight's point at #252 (comment) still holds true. I haven't looked to see what all the contributions were, but I think it's safe to assume that all the contributors are Wikimedians who contributed with the intent that their work would eventually make its way onto Wikimedia sites, where it would be CC-BY-SA 3.0 licensed. Yes, technically there was no license statement in the repository, but I think the intent and process is pretty clear for a project like this.

[siddharthvp]

Agree with @legoktm that we should be good with this.

It's worth noting CC-BY-SA is not an OSI-approved license, but relicensing the project under a proper license would be hard as many of the early contributors are no longer active.