add some info on site docs

README.md

@@ -39,6 +39,13 @@ vodsl can be installed by following [these instructions](https://github.com/ivoa
 The use cases for the model are tested by writing [java test cases](./src/test/java/org/ivoa/dm/proposal/prop) against the auto-generated code. It is always useful
 to add a test for a new observatory to ensure that the model meets its requirements.
 
+Model site documentation (in [the docs directory](docs)) requires that [graphviz](https://gitlab.com/graphviz/graphviz) and [yq](https://github.com/mikefarah/yq) are installed - it can be
+tested with
+
+```
+gradle testSite
+```
+
 You should follow the usual [github flow](https://docs.github.com/en/get-started/quickstart/github-flow) for making actual changes
 to the model or standard document. It is also possible to contribute suggestions and errata by opening an [issue](../../issues) 
 or less formally a [discussion](../../discussions).