Add a contributing file

The exact sequence of commands to be able to run the tests wasn't obvious
to me so I'm assumiing others could use a hand.

I used the factory_girl_rails gem's file as a starting point.

CONTRIBUTING.md

@@ -0,0 +1,33 @@
+# Contributing
+
+Fork, then clone the repo:
+
+```
+git clone git@github.com:your-username/swagger_rails.git
+cd swagger_rails
+```
+
+Set up your machine:
+
+```
+bundle
+cd spec/dummy
+bundle exec rake db:setup
+cd -
+```
+
+Make sure the tests pass:
+
+```
+bundle exec rspec
+```
+
+Make your change. Add tests for your change. Make the tests pass:
+
+```
+bundle exec rspec
+```
+
+Push to your fork and [submit a pull request][pr].
+
+[pr]: https://github.com/domaindrivendev/swagger_rails/compare/