document SKIP_NOTEBOOKS

CONTRIBUTING.rst

@@ -111,6 +111,12 @@ Ready to contribute? Here's how to set up `xscen` for local development.
     $ cd docs/
     $ make html
 
+.. note::
+
+    When building the documentation, the default behaviour is to evaluate notebooks ('nbsphinx_execute = "always"'), rather than simply parse the content ('nbsphinx_execute = "never"'). Due to their complexity, this can sometimes be a very computationally demanding task and should only be performed when necessary (i.e.: when the notebooks have been modified).
+
+    In order to speed up documentation builds, setting a value for the environment variable "SKIP_NOTEBOOKS" (e.g. "$ export SKIP_NOTEBOOKS=1") will prevent the notebooks from being evaluated on all subsequent "$ tox -e docs" or "$ make docs" invocations.
+
 8. Submit a pull request through the GitHub website.
 
 Pull Request Guidelines