Update contribution guidelines

CONTRIBUTING.md

@@ -1,9 +1,10 @@
-1. Fork it ( <https://github.com/rdf-elixir/jsonld-ex/fork> )
+1. Fork it (<https://github.com/rdf-elixir/jsonld-ex/fork>)
 2. Create your feature branch (`git checkout -b my-new-feature`)
 3. Make your changes, with new passing tests. Follow this [style guide].
-4. Execute all tests.
-5. Commit your changes (`git commit -am 'Add some feature'`)
-6. Push to the branch (`git push origin my-new-feature`)
-7. Create a new Pull Request
+4. Execute all tests (`mix test`)
+5. Apply the mix formatter (`mix format`)
+6. Commit your changes (`git commit -am 'Add some feature'`)
+7. Push to the branch (`git push origin my-new-feature`)
+8. Create a new Pull Request
 
 [style guide]:  https://github.com/christopheradams/elixir_style_guide