doc/developer: Add web links for all suggested licenses

doc/developer.texi

@@ -248,8 +248,11 @@ For Emacs, add these roughly equivalent lines to your @file{.emacs.d/init.el}:
 
 @enumerate
 @item
-   Contributions should be licensed under the LGPL 2.1, including an
-   "or any later version" clause, or the MIT license.  GPL 2 including
+   Contributions should be licensed under the
+   @uref{http://www.gnu.org/licenses/lgpl-2.1.html, LGPL 2.1},
+   including an "or any later version" clause, or the
+   @uref{http://mit-license.org/, MIT license}.
+   @uref{http://www.gnu.org/licenses/gpl-2.0.html, GPL 2} including
    an "or any later version" clause is also acceptable, but LGPL is
    preferred.
 @item