diff --git a/docs/source/documenting.rst b/docs/source/documenting.rst
index bc74dae6..a43dae7d 100644
--- a/docs/source/documenting.rst
+++ b/docs/source/documenting.rst
@@ -25,7 +25,11 @@ To build the manual (``man``) pages::
 
 The generated man pages will be in ``docs/man/``.
 
-To build the documentation diagrams::
+To build the documentation diagrams, extra system dependencies are needed::
+
+    apt-get install graphviz
+
+You can then build the diagrams with::
 
     cd docs/ && make umlsvg
 
@@ -46,4 +50,4 @@ They are included in most distributions. In Debian install them running::
 .. _recommonmark: https://recommonmark.readthedocs.io/
 .. _Pylint: https://www.pylint.org/
 .. _Tex: https://www.tug.org/texlive/acquire.html
-.. _dvipng: https://www.nongnu.org/dvipng/
\ No newline at end of file
+.. _dvipng: https://www.nongnu.org/dvipng/