Split LICENSE files

[maxdeviant]

This PR splits the LICENSE file out into two files:

• LICENSE for the free license
• LICENSE_THIRDPARTY for the notice of adaptation from the Haskell libraries

licensee will now correctly detect the contents of the LICENSE file:

λ licensee detect LICENSE
License:        BSD-3-Clause
Matched files:  LICENSE
LICENSE:
  Content hash:  a961b19cc6921d510e29a13b0ba1a826fcffe41c
  Attribution:   Copyright 2018 PureScript
  Confidence:    100.00%
  Matcher:       Licensee::Matchers::Exact
  License:       BSD-3-Clause

Motivation

free is currently in the PureScript package set and will need to have this license issue addressed in order to remain in the package set after the move to the new PureScript registry.

See purescript/registry#250 for more details.