README: Add link to documentation

This commit adds a link to the documentation on pythonhosted.org.

Author: DirectXMan12

README.txt

@@ -21,6 +21,9 @@ Python-GSSAPI provides both low-level and high level wrappers around the GSSAPI
 C libraries.  While it focuses on the Kerberos mechanism, it should also be
 useable with other GSSAPI mechanisms.
 
+Documentation for the latest released version can be found at
+`https://pythonhosted.org/gssapi <https://pythonhosted.org/gssapi>`_.
+
 Requirements
 ============