HACKING.rst: Add a note about arpy being required for docs
diff --git a/HACKING.rst b/HACKING.rst
index e89e282..16d9a1d 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -137,6 +137,12 @@
Furthermore, the documentation build requires that BuildStream itself
be installed.
+You will also need any dependencies required for plugins, even if
+these aren't otherwise hard dependencies. This currently applies to
+``arpy``::
+
+ pip install --user arpy
+
To build the documentation, just run the following::
make -C doc
