add @dantownsend suggestions

Author: sinisaos

docs/source/contributing/index.rst

@@ -113,7 +113,8 @@ Contributing to the docs
 
 The docs are written using Sphinx. To get them running locally:
 
-* Install the requirements: ``pip install -r requirements/readthedocs-requirements.txt``
+* Make sure the main requirements are installed: ``pip install -r requirements/requirements.txt``
+* Install the documentation requirements: ``pip install -r requirements/doc-requirements.txt``
 * ``cd docs``
 * Do an initial build of the docs: ``make html``
 * Serve the docs: ``python serve_docs.py``