Add contributing guide and suggest swagger_engine

CONTRIBUTING.md

+# Contributing
+
+## Running tests
+
+The `make_site.sh` script will create a test site for a specific version of
+Rails and run the tests:
+```
+RAILS_VERSION=4.2.0
+./make_site.sh
+```
+
+Once the test site is created you can just re-run the tests:
+```
+bundle exec rspec
+```

README.md

@@ -61,4 +61,5 @@ bundle exec rake swagger
 ```
 
 Now you can use Swagger UI or the renderer of your choice to display the
-formatted documentation.
+formatted documentation. [swagger_engine](https://github.com/batdevis/swagger_engine)
+works pretty well and supports multiple documents.