add README about what can be found here

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@38809 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/external/README.license

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+
+
+In the following directories you will find code which was not
+written by The GRASS Development Team.
+
+Parts of GRASS are not copyright by The GRASS Development Team.
+The original authors hold the copyrights and you have to abide
+to their licensing terms where noted.
+(Keep in mind that code linking into GRASS can only be distributed
+if compatible with the GPL.)
+
+Specifically the source code below this directory is not necessarily
+licensed under the GPL. Check the READMEs and source code header
+comments carefully.
+
+