Add contributing guidelines.

CONTRIBUTING.md

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+Contribute to Guard::Cucumber
+=============================
+
+File an issue
+-------------
+
+You can report bugs and feature requests to [GitHub Issues](https://github.com/netzpirat/guard-cucumber/issues).
+
+**Please don't ask question in the issue tracker**, instead ask them on at Stack Overflow and use the
+[guard](http://stackoverflow.com/questions/tagged/guard) tag.
+
+Try to figure out where the issue belongs to: Is it an issue with Guard itself or with a Guard plugin you're
+using?
+
+When you file a bug, please try to follow these simple rules if applicable:
+
+* Make sure you've read the README carefully.
+* Make sure you run Guard with `bundle exec` first.
+* Add debug information to the issue by running Guard with the `--debug` option.
+* Add your `Guardfile` and `Gemfile` to the issue.
+* Make sure that the issue is reproducible with your description.
+
+**It's most likely that your bug gets resolved faster if you provide as much information as possible!**
+
+Development
+-----------
+
+* Documentation hosted at [RubyDoc](http://rubydoc.info/github/netzpirat/guard-cucumber/master/frames).
+* Source hosted at [GitHub](https://github.com/netzpirat/guard-cucumber).
+
+Pull requests are very welcome! Please try to follow these simple rules if applicable:
+
+* Please create a topic branch for every separate change you make.
+* Make sure your patches are well tested. All specs run with `rake spec:portability` must pass.
+* Update the [Yard](http://yardoc.org/) documentation.
+* Update the [README](https://github.com/netzpirat/guard-cucumber/blob/master/README.md).
+* Update the [CHANGELOG](https://github.com/netzpirat/guard-cucumber/blob/master/CHANGELOG.md) for noteworthy changes.
+* Please **do not change** the version number.
+
+For questions please join us in our [Google group](http://groups.google.com/group/guard-dev) or on
+`#guard` (irc.freenode.net).