Update test/acceptance/README.rst

test/acceptance/README.rst

 Selenium Tests
 ==============
 
-These are UI-level acceptance tests designed to be executed on an edx-platform sandbox instance.
+These are UI-level acceptance and accessibility tests designed to be run against an edx-platform sandbox instance.
 
 The tests use the ``bok-choy`` library.  For a tutorial, see `here`__.
 
@@ -18,10 +18,23 @@ To use the tests:
     pip install -r requirements/test-acceptance.txt
 
 
-2. Run the tests
+2. Prepare for tests
 
 .. code:: bash
 
     cd edx-ora2/test/acceptance
     export BASE_URL=https://{USER}:{PASSWORD}@example.com
+
+3. Run the tests
+
+To run the acceptance tests:
+    
+.. code:: bash
+
     python tests.py
+
+To run the accessibility tests, which must be run with phantomjs as the browser:
+    
+.. code:: bash
+
+    SELENIUM_BROWSER=phantomjs python accessibility.py