DOC: Clarify requirements installation for docs autobuild

Small change to indicate that the `requirements.txt` installation step
listed in "Option 1" for building docs is also required for autobuilding
in "Option 2".

Documentation/docs/README.md

@@ -39,6 +39,7 @@ To automatically rebuild the website with any input markdown changes and serve
 the result, use [sphinx-autobuild]
 
 ```sh
+pip install -r requirements.txt
 pip install sphinx-autobuild
 ```