Create CONTRIBUTING.md

CONTRIBUTING.md

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+Contributions are much appreciated.  
+Please open a pull request or add an issue to discuss what you intend to work on.  
+If the pull requests passes the CI and conforms to the existing style of specs, it will be merged.
+
+### Creating files for currently unspecified modules or classes
+
+If you want to create specs for a module or class and the files do not exist yet,
+you should use `mkspec`, part of [MSpec](http://github.com/ruby/mspec):
+
+    $ path/to/mspec/bin/mkspec -h
+
+Specify `core` or `library` as the `base`.