Copy over PlasmaPy's license

...so that we can include content from PlasmaPy's Contributor Guide.

Author: namurphy

licenses/PlasmaPy_license.md

@@ -0,0 +1,28 @@
+BSD 3-Clause License
+
+Copyright (c) 2015–2025, PlasmaPy Developers.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+* Neither the name of PlasmaPy nor the names of its contributors may be used
+  to endorse or promote products derived from this software without specific
+  prior written permission.
+
+This software is provided by the copyright holders and contributors "as is"
+and any express or implied warranties, including, but not limited to, the
+implied warranties of merchantability and fitness for a particular purpose are
+disclaimed. In no event shall the copyright holder or contributors be liable
+for any direct, indirect, incidental, special, exemplary, or consequential
+damages (including, but not limited to, procurement of substitute goods or
+services; loss of use, data, or profits; or business interruption) however
+caused and on any theory of liability, whether in contract, strict liability,
+or tort (including negligence or otherwise) arising in any way out of the use
+of this software, even if advised of the possibility of such damage.