Split LICENSE files

[maxdeviant]

This PR splits the LICENSE file out into two files:

• LICENSE for the foreign-generic license
• LICENSE_THIRDPARTY for the notice of adaptation from foreign

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

λ licensee detect LICENSE
License:        MIT
Matched files:  LICENSE
LICENSE:
  Content hash:  4c2c763d64bbc7ef2e58b0ec6d06d90cee9755c9
  Attribution:   Copyright (c) 2017 Phil Freeman
  Confidence:    100.00%
  Matcher:       Licensee::Matchers::Exact
  License:       MIT

Motivation

foreign-generic 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.