Added contributing guidelines and licensing information

LICENSE

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+MIT License
+
+Copyright (c) 2017 Aravind Krishnamoorthy
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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 # xtal
 
 xtal is an umbrella package for various tools used to manipulate atomic trajectories
+
+
+## Contributing
+
+Contributing to xtal is easy, simply fork the repo, add you code and documentation and submit a pull request. For consistency with the existing codebase, please follow the [Google Python style guide](https://google.github.io/styleguide/pyguide.html) as closely as possible.
+
+
+## License
+
+The passive film model source code and documentation are distributed 'as is' under the [MIT License](LICENSE).