Add a LICENSE.txt file explaining the licensing of the build files and of the built toolchain

LICENSE.txt

@@ -0,0 +1,21 @@
+The llvm-mingw project (the scripts for building and assembling a
+toolchain, wrappers for executing tools and testcases) is licensed under
+the ISC license.
+
+These licenses only cover the components provided by llvm-mingw directly.
+The final built toolchain is covered by the licenses of the individual
+external projects, primarily LLVM's Apache 2 license with LLVM exceptions.
+
+The ISC license:
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.