mention third_party in LICENSE

Author: kripken

LICENSE

@@ -96,3 +96,7 @@ in accordance with the terms of the MIT license. Node's license follows:
 The musl libc project is bundled in this repo, and it has the MIT license, see
 system/lib/libc/musl/COPYRIGHT
 
+The third_party/ subdirectory contains code with other licenses. None of it is
+used by default, but certain options use it (e.g., the optional closure compiler
+flag will run closure compiler from third_party/).
+