Update version and license

Added LICENSE.txt and GPLv3.txt. Also updated libco documentation.

After discussion with byuu, libco gets a more specific ISC license
to match nall, ruby and hiro. higan, as clarified in LICENSE.txt,
continues to be GPL version 3 only (no "or later" clause).
3 jobs for master in 29 minutes and 36 seconds (queued for 9 minutes and 45 seconds)
Status Job ID Name Coverage
  Test
passed #37528123
libretro-test

00:03:46

passed #37528121
linux-x86_64-binaries

00:23:36

passed #37528122
windows-x86_64-binaries

00:14:13