Update contributing.md (#385)

small update for contributing.md to include what packages to install and
how to lint.

CONTRIBUTING.md

@@ -2,6 +2,10 @@
 We want to make contributing to this project as easy and transparent as
 possible.
 
+## Setup
+```
+pip install -r dev-requirements.txt
+```
 
 ## Pull Requests
 We actively welcome your pull requests.
@@ -10,7 +14,7 @@ We actively welcome your pull requests.
 2. If you've added code that should be tested, add tests.
 3. If you've changed APIs, update the documentation.
 4. Ensure the test suite passes.
-5. Make sure your code lints.
+5. Make sure your code lints (`pre-commit run --all-files`).
 6. If you haven't already, complete the Contributor License Agreement ("CLA").
 
 ## Contributor License Agreement ("CLA")