Adding custom tags to docs

README.md

@@ -38,6 +38,11 @@ This does the following:
 - Set the ``always_yes`` config option for conda to ``true`` so that you don't need to include ``--yes``.
 - Register the specified channels, or if not stated the ``astropy``, ``astropy-ci-extras``, and ``openastronomy`` channels.
 - ``export PYTHONIOENCODING=UTF8``
+- Supports custom skip tags included in the commit message that are not yet
+  natively provided by Travis. To skip the travis build: [skip travis] or
+  [travis skip]. To run only the docs build: [build docs] or [docs
+  only]. The latter requires ``SETUP_CMD`` (see below) to be set to
+  "build_docs" or "build_sphinx".
 
 Following this, various dependencies are installed depending on the following
 environment variables