licenses: comment about two versions of MIT

I decided to follow spdx.org and not to differentiate those two.
Packages would often have the wrong version anyway.

lib/licenses.nix

@@ -194,6 +194,8 @@ rec {
     fullName = "Lucent Public License v1.02";
   };
 
+  # spdx.org does not (yet) differentiate between the X11 and Expat versions
+  # for details see http://en.wikipedia.org/wiki/MIT_License#Various_versions
   mit = spdx {
     shortName = "MIT";
     fullName = "MIT License";