Add license file

LICENSE

+The dune-curvedgrid library, headers and test programs are copyrighted free software. You
+can use, modify and/or redistribute it under the terms of either one of the two
+following licenses:
+
+* The GNU Lesser General Public License as published by the Free Software
+  Foundation, either Version 3 of the license or (at your option) any later
+  version. You can find a copy of the GNU Lesser General Public License, Version 3,
+  at <http://www.gnu.org/licenses/lgpl-3.0>.
+
+* Version 2 of the GNU General Public License as published by the Free Software
+  Foundation, with the following special exception for linking and compiling
+  against the dune-curvedgrid library, the so-called "runtime exception":
+
+    As a special exception, you may use the dune-curvedgrid source files as part of a
+    software library or application without restriction.  Specifically, if other
+    files instantiate templates or use macros or inline functions from one or
+    more of the dune-curvedgrid source files, or you compile one or more of the dune-curvedgrid
+    source files and link them with other files to produce an executable, this
+    does not by itself cause the resulting executable to be covered by the GNU
+    General Public License. This exception does not however invalidate any
+    other reasons why the executable file might be covered by the GNU General
+    Public License.
+
+  This license is intended to be similar to the GNU Lesser General Public
+  License, Version 2, which by itself isn't suitable for a template library. You
+  can find a copy of the GNU General Public License, Version 2,
+  at <http://www.gnu.org/licenses/gpl-2.0>.
+