doc: add BLAST+ license

software/blast+/LICENSE

@@ -0,0 +1,20 @@
+                            PUBLIC DOMAIN NOTICE
+               National Center for Biotechnology Information
+
+  This software/database is a "United States Government Work" under the
+  terms of the United States Copyright Act.  It was written as part of
+  the author's official duties as a United States Government employee and
+  thus cannot be copyrighted.  This software/database is freely available
+  to the public for use. The National Library of Medicine and the U.S.
+  Government have not placed any restriction on its use or reproduction.
+
+  Although all reasonable efforts have been taken to ensure the accuracy
+  and reliability of the software and data, the NLM and the U.S.
+  Government do not and cannot warrant the performance or results that
+  may be obtained by using this software or data. The NLM and the U.S.
+  Government disclaim all warranties, express or implied, including
+  warranties of performance, merchantability or fitness for any particular
+  purpose.
+
+  Please cite the author in any work or product based on this material.
+