Add instructions for setting up optional dependencies

docs_rst/contributing.rst

@@ -36,7 +36,12 @@ Direct contributions to pymatgen main distribution
 
    Note that the entire Github repo is fairly large because of the presence of test files, but these are absolutely
    necessary for rigorous testing of the code.
-5. It is highly recommended you install all the optional dependencies as well.
+5. It is highly recommended you install all the optional dependencies as well::
+
+      pip install -r requirements.txt
+      pip install -r requirements-optional.txt
+      pip install -r requirements-dev.txt
+
 6. Code (see `Coding Guidelines`_). Commit early and commit often. Keep your code up to date. You need to add the main
    repository to the list of your remotes. Let's name the upstream repo as mpmaster (materialsproject master)::