Add note about older versions.

Author: anthony-tuininga

doc/src/index.rst

@@ -5,7 +5,7 @@ Welcome to cx_Oracle's documentation!
 **cx_Oracle** is a module that enables access to Oracle Database and conforms
 to the Python database API specification. This module is currently tested
 against Oracle Client 19c, 18c, 12c, and 11.2, and Python 3.5, 3.6, 3.7 and
-3.8.
+3.8.  Older versions of cx_Oracle may be used with previous Python releases.
 
 **cx_Oracle** is distributed under an open-source :ref:`license <license>`
 (the BSD license). A detailed description of cx_Oracle changes can be found in