update building docs instructions in contributing guide (#1235)

diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 40a74c785586ca869d49c322c4b966a7112ea2c7..2b6abe83d4d0a90a9a5fc6a532a9aa57d0e2c9a1 100644 (file)
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -207,6 +207,12 @@ Read more about `coverage <https://coverage.readthedocs.io>`__.
 Building the docs
 ~~~~~~~~~~~~~~~~~
 
+Install the documentation dependencies if you have not already done so.
+
+.. code-block:: text
+
+    $ pip install -r requirements/docs.txt
+
 Build the docs in the ``docs`` directory using Sphinx.
 
 .. code-block:: text