updated python install instructions

_includes/install_instructions/python.html

@@ -43,6 +43,7 @@ https://jupyter-notebook.readthedocs.io/en/stable/notebook.html#browser-compatib
             Verify the installation by opening the Terminal app and type 'python3 --version'.
             The output should be something like 'Python 3.x.x'
           </li>
+          <li>If the version < 3 or 'python3' is not found, please follow <a href="https://docs.python-guide.org/starting/install3/osx/">The Hitchhiker's Guide to Python</a></li>
         </ol>
       </article>
       <article role="tabpanel" class="tab-pane" id="python-linux">