README: Add instructions for building API documentation

README.md

@@ -15,3 +15,12 @@ use:
 
 Where `FILENAME` is a yaml file describing the input. There is an
 example in `examples/example.yaml`.
+
+## API documentation
+
+The documentation for the internal API can be built and opened in the
+default browser with:
+
+    stack haddock --open ma
+
+This can help to understand the code structure.