Split LICENSE file into separate LICENSE and AUTHORS files.

[tswast]

The GitHub robot (Ruby Licenseee Gem) isn't properly detecting the license as BSD simplified.

Seems to me that the About the Copyright Holders and Our Copyright Policy sections could be pulled into an AUTHORS.md file. The LICENSE file could then be the generic BSD license, which is more easily detected by scrapers such as GitHub's logic.

[parthea]

cc @jreback

[jreback]

you could do this

the license was just copied directly from pandas (so could submit a pr there as well)