Add instruction to run all tests in CONTRIBUTING (#1150)

This instruction existed before the last update to the document.

CONTRIBUTING.md

@@ -140,6 +140,14 @@ Instead, you need to say
 bundle exec appraisal 5.1 rspec spec/acceptance/active_model_integration_spec.rb
 ```
 
+### Run all tests
+
+In order to run all tests, simply run:
+
+```
+bundle exec rake
+```
+
 ## Managing your branch
 
 * Use well-crafted commit messages, providing context if possible. [tpope's