Add instructions

utils/haddock/doc/diagrams/README.md

+# Diagrams
+
+The diagrams are generated using Calligraphy[^1].
+
+To re-generate the diagrams, run the following command on the distributed versions of haddock-library and haddock-api from Hackage:
+
+```
+# In the haddock-library directory
+$ calligraphy --output-mermaid ~/haddock-library.mmd --collapse-modules
+# In teh haddock-api directory
+$ calligraphy --output-mermaid ~/haddock-api.mmd --collapse-modules
+```
+
+[1]: https://github.com/jonascarpay/calligraphy#readme