Added License file

git-svn-id: https://epanet.svn.sourceforge.net/svnroot/epanet/BASE/trunk@378 c320cabd-cc23-0410-96d8-e60fbf53ed7f

doc/LICENSE.txt

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+The EPANET software was developed by Lewis Rossman (US-EPA).
+
+This public-domain software developed by the U.S. EPA,
+National Risk Management Research Laboratory in Cincinnati, Ohio.
+Although these programs have been used by the U.S. EPA, no warranty,
+expressed or implied, is made by the U.S. EPA as to the accuracy and
+functioning of the programs and related program material, nor shall the
+fact of distribution constitute any such warranty, and no responsibility
+is assumed by the U.S. EPA in connection therewith.