doc: Document the staged build

README.md

@@ -18,3 +18,34 @@ You could e.g. host the files locally with the built-in Python webserver:
 ```sh
 $ cd build/doc && python -m http.server
 ```
+
+### Multi stage
+You can also use `doc-gen4` in multiple separate stages to generate the whole documentation.
+For example `mathlib4` consists out of 4 modules, the 3 Lean compiler ones and itself:
+- `Init`
+- `Std`
+- `Lean`
+- `Mathlib`
+The first stage in the build is:
+```sh
+$ doc-gen4 init Mathlib
+```
+We already have to pass the `Mathlib` top level module here so it can generate the
+navbar on the left hand side properly in the index and 404 HTML pages.
+Next we can run the actual build stages:
+1. `doc-gen4 single Init Mathlib`
+2. `doc-gen4 single Std Mathlib`
+3. `doc-gen4 single Lean Mathlib`
+4. `doc-gen4 single Mathlib Mathlib`
+We have to pass `Mathlib` here again for the same reason, the single command
+will only generate documentation for its first argument module. Furthermore
+one can use the `--ink` flag here to also generate LeanInk documentation in addition.
+The last stage is the finalize one which zips up some information relevant for
+the search:
+```sh
+$ doc-gen4 finalize
+```
+Now `build/doc` should contain the same files with the same context as if one had run
+```
+$ doc-gen4 Mathlib
+```