release 3.20.0: docs

Author: aboudreault

docs.yaml

@@ -22,6 +22,8 @@ sections:
       # build extensions like libev
       CASS_DRIVER_NO_CYTHON=1 python setup.py build_ext --inplace --force
 versions:
+  - name: '3.20'
+    ref: d30d166f
   - name: '3.19'
     ref: ac2471f9
   - name: '3.18'