Add PlasmaPy's license so we can adapt contributor guide

Author: namurphy

licenses/PlasmaPy_license.md

@@ -5,14 +5,14 @@ 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
+- 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,
+- 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
+- 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.