Further cleanup of contributing.md

Author: juliannguyen4

CONTRIBUTING.md

@@ -13,14 +13,6 @@ To install dependencies for development:
 pip install -r requirements.txt
 ```
 
-## Documentation
-
-To build documentation:
-```
-cd docs/
-sphinx-build -b html . htmldir
-```
-
 ## Unit testing
 
 The tests use a locally built and installed module for this code:
@@ -45,6 +37,14 @@ Then run the tests:
 python3 tests/test.py
 ```
 
+## Documentation
+
+To build documentation:
+```
+cd docs/
+sphinx-build -b html . htmldir -W
+```
+
 ## Codestyle
 
 This code uses [flake8](https://github.com/pycqa/flake8) for codestyle checking.