doc: Clarify licensing issues arising from external libraries

LICENSE

@@ -1,5 +1,5 @@
 Libav:
-------
+======
 
 Most files in Libav are under the GNU Lesser General Public License version 2.1
 or later (LGPL v2.1+). Read the file COPYING.LGPLv2.1 for details. Some other
@@ -38,18 +38,29 @@ for you. Read the file COPYING.LGPLv3 or, if you have enabled GPL parts,
 COPYING.GPLv3 to learn the exact legal terms that apply in this case.
 
 
-external libraries:
--------------------
+external libraries
+==================
 
-Some external libraries, e.g. libx264, are under GPL and can be used in
-conjunction with Libav. They require --enable-gpl to be passed to configure
-as well.
+Libav can be combined with a number of external libraries, which sometimes
+affect the licensing of binaries resulting from the combination.
 
-The OpenCORE external libraries are under the Apache License 2.0. That license
-is incompatible with the LGPL v2.1 and the GPL v2, but not with version 3 of
-those licenses. So to combine the OpenCORE libraries with Libav, the license
-version needs to be upgraded by passing --enable-version3 to configure.
+compatible libraries
+--------------------
 
-The nonfree external library libfaac can be hooked up in Libav. You need to
-pass --enable-nonfree to configure to enable it. Employ this option with care
-as Libav then becomes nonfree and unredistributable.
+The libcdio, libx264, libxavs and libxvid libraries are under GPL. When
+combining them with Libav, Libav needs to be licensed as GPL as well by
+passing --enable-gpl to configure.
+
+The OpenCORE and VisualOn libraries are under the Apache License 2.0. That
+license is incompatible with the LGPL v2.1 and the GPL v2, but not with
+version 3 of those licenses. So to combine these libraries with Libav, the
+license version needs to be upgraded by passing --enable-version3 to configure.
+
+incompatible libraries
+----------------------
+
+The Fraunhofer AAC library, FAAC and OpenSSL are under licenses incompatible
+with all (L)GPL versions. Thus, unfortunately, since both licenses cannot be
+satisfied simultaneously, binaries resulting from the combination of Libav
+with these libraries are nonfree und unredistributable. If you wish to enable
+any of these libraries nonetheless, pass --enable-nonfree to configure.