DOC: add example for DataFrame.resample: keywords on and level
 - [ ] closes #xxxx

add examples for DataFrame specific keywords: on and level.
This can possibly fix issue #14843 

See attached picture for details

![screenshot - 080317 - 21 34 00](https://cloud.githubusercontent.com/assets/10461030/23722782/fad80f30-0446-11e7-8ebe-e0caff835f01.png)

