Add basic instructions to build the docs

README.md

@@ -9,3 +9,18 @@ offline copy of all Qubes documentation rather than relying solely on the Web.
 
 To contribute, please see [how to edit the
 documentation](https://www.qubes-os.org/doc/how-to-edit-the-documentation/).
+
+## How to build the documentation
+
+In a qube, after cloning or downloading this repo, do:
+
+    python3 -m venv .venv
+    source .venv/bin/activate
+    pip install -r requirements.txt
+
+Then, build the documentation using any sphinx builder, i.e. with HTML:
+
+    sphinx-build -M html . build
+
+The HTML version of the documentation is now accessible in `build/html`
+