Add contributing guide [ci skip]

Closes #251.

Author: jonleighton

CONTRIBUTING.md

@@ -0,0 +1,32 @@
+# Contributing guide
+
+## Getting set up
+
+Check out the code and run `bundle install` as usual.
+
+## Running tests
+
+Running `rake` will run all tests. There are both unit tests and
+acceptance tests. You can run them individually with `rake test:unit` or
+`rake test:acceptance`.
+
+If one doesn't already exist, the acceptance tests will generate a dummy
+Rails app in `test/apps/`. On each test run, the dummy app is copied to
+`test/apps/tmp/` so that any changes won't affect the pre-generated app
+(this saves us having to regenerate the app on each run).
+
+If tests are failing and you don't know why, it might be that the
+pre-generated app has become inconsistent in some way. In that case the
+best solution is to purge it with `rm -rf test/apps/*` and then run the
+acceptance tests again, which will generate a new app.
+
+## Testing different Rails versions
+
+You can set the `RAILS_VERSION` environment variable:
+
+```
+$ RAILS_VERSION="~> 3.2.0" rake test:acceptance
+```
+
+The apps in `test/apps` will be named based on the rails version and the
+spring version.