Add missing command from CONTRIBUTING guide

Running the rspec tests require first running the `spec_prep` rake
command. This wasn't documented. An argument can be made for it being
'common knowledge', but that goes out the window with the 'all in one
solution' for using bundler.

.github/CONTRIBUTING.md

@@ -163,6 +163,11 @@ environment variable:
 BUNDLE_JOBS="$(nproc)"
 ```
 
+Prepare to run tests:
+```sh
+bundle exec rake spec_prep
+```
+
 ### Note for OS X users
 
 `nproc` isn't a valid command under OS x. As an alternative, you can do: