Add LICENSE.md

I didn't realize this was missing until PackageEvaluator said so, but I guess it's pretty important. I just copied the file from DataFrames and changed the first line.

Sorry to spam everyone, but @johnmyleswhite, @garborg, @dcjones, @GlenHertz, @kmsquire, @StefanKarpinski, can you all confirm you license your contributions to DataArrays under this license?