Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -152,7 +152,7 @@ So the steps for committing to the master branch are:
 
 3. Ensure all unit tests pass.
 
-4. In the Python repo, ensure that the documentation is generated without errors. First, run `pip install sphinx sphinx_rtd_theme`. Then, from within the subfolder `doc`, run the command `make html` (or `make.bat html` on Windows), ensure there are no errors, and inspect the documentation it generates in the folder `build`.
+4. In the Python repo, ensure that the documentation is generated without errors. First, run `pip install sphinx sphinx_rtd_theme`. This installs [Sphinx](https://www.sphinx-doc.org/en/master/), which is the most well-supported documentation generator for Python. (It's not very friendly, the syntax for things like links in docstrings is awkward, but it's well supported, so we use it.) Then, from within the subfolder `doc`, run the command `make html` (or `make.bat html` on Windows), ensure there are no errors, and inspect the documentation it generates in the folder `_build`.
 
 5. Create a PR to merge changes from dev into master.