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 v105tr1 in 22 minutes and 46 seconds (queued for 32 minutes and 45 seconds)
Status Job ID Name Coverage
  Test
passed #37528190
libretro-test

00:04:54

passed #37528188
linux-x86_64-binaries

00:12:39

passed #37528189
windows-x86_64-binaries

00:16:24