Update CONTRIBUTING guidelines

Author: marcelotto

CONTRIBUTING.md

@@ -1,7 +1,7 @@
 1. Fork it ( <https://github.com/rdf-elixir/sparql_client/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.
+4. Execute all tests and quality checks (`mix check && mix dialyzer`).
 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