Add API docs instruction to README.md

Author: stes

README.md

@@ -100,6 +100,18 @@ A number of labs are currently adopting DataJoint and we are quickly getting the
 * https://tutorials.datajoint.io -- step-by-step tutorials
 * https://catalog.datajoint.io -- catalog of example pipelines
 
+### API docs
+
+The API documentation can be built using sphinx by running
+
+``` bash
+pip install sphinx sphinx_rtd_theme 
+(cd docs-api/sphinx && make html)
+```
+
+Generated docs are written to `docs-api/docs/html/index.html`.
+More details in [docs-api/README.md](docs-api/README.md).
+
 ## Running Tests Locally