Update CONTRIBUTING.md

Author: patrick-kidger

CONTRIBUTING.md

@@ -4,6 +4,8 @@ Contributions (pull requests) are very welcome! Here's how to get started.
 
 ---
 
+**Getting started**
+
 First fork the library on GitHub.
 
 Then clone and install the library in development mode:
@@ -23,6 +25,10 @@ pre-commit install
 
 These hooks use Black and isort to format the code, and flake8 to lint it.
 
+---
+
+**If you're making changes to the code:**
+
 Now make your changes. Make sure to include additional tests if necessary.
 
 Next verify the tests all pass:
@@ -39,3 +45,21 @@ git push
 ```
 
 Finally, open a pull request on GitHub!
+
+---
+
+**If you're making changes to the documentation:**
+
+Make your changes. You can then build the documentation by doing
+
+```bash
+pip install -r docs/requirements.txt
+mkdocs serve
+```
+Then doing `Control-C`, and running:
+```
+mkdocs serve
+```
+(So you run `mkdocs serve` twice.)
+
+You can then see your local copy of the documentation by navigating to `localhost:8000` in a web browser.