Adding approved disclaimer

DISCLAIMER.md

+Disclaimer
+==========
+
+This software has been approved for release by the U.S. Geological Survey
+(USGS). Although the software has been subjected to rigorous review, the USGS
+reserves the right to update the software as needed pursuant to further analysis
+and review. No warranty, expressed or implied, is made by the USGS or the U.S.
+Government as to the functionality of the software and related material nor
+shall the fact of release constitute any such warranty. Furthermore, the
+software is released on condition that neither the USGS nor the U.S. Government
+shall be held liable for any damages resulting from its authorized or
+unauthorized use.