-The new source format is [Sphinx](http://sphinx-doc.org), a documentation
-build tool written in [Python](http://python.org), which you will need to
-install if you want to build the document. You can find the complete
-installation instructions
-[here](http://www.sphinx-doc.org/en/stable/install.html).
+As well as these document sources, you will need:
+
+* Sphinx. You can find the complete installation instructions
+ [here](http://www.sphinx-doc.org/en/stable/install.html).
+
+* Sphinx makes use of [Blockdiag](https://pypi.python.org/pypi/blockdiag)
+ for some of the diagrams; you'll need that too.
+
+If you have `pip`, this command will be all you need:
+
+ pip install sphinx blockdiag