doc: Basic usage in README

README.md

@@ -1,2 +1,15 @@
 # doc-gen4
 Document Generator for Lean 4
+
+## Usage
+You can call `doc-gen4` from the top of a Lake project like this:
+```sh
+$ /path/to/doc-gen4 Module
+```
+Where `Module` is one or more of the top level modules you want to document.
+The tool will then proceed to compile the project using lake (if that hasn't happened yet),
+analyze it and put the result in `./build/doc`.
+You could e.g. host the files locally with the built-in Python webserver:
+```sh
+$ cd build/doc && python -m http.server
+```