ENH: added rsplit to StringMethods
as a part of https://github.com/pydata/pandas/issues/9111 

cc @sinhrks 

Thanks! I think this is OK for 0.16.2 as a follow-up of string enh if there is no objections. Could you update:
- Release note
- `text.rst` and `api.rst`
 @sinhrks it's updated, please take a look 
 thanks!
