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)


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15627?src=pr&el=h1) Report
> Merging [#15627](https://codecov.io/gh/pandas-dev/pandas/pull/15627?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/54e71a74c81ecefb55bb35934b75f4cd1fb3ded1?src=pr&el=desc) will **decrease** coverage by `-0.03%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15627      +/-   ##
==========================================
- Coverage   91.01%   90.99%   -0.03%     
==========================================
  Files         143      143              
  Lines       49301    49301              
==========================================
- Hits        44873    44863      -10     
- Misses       4428     4438      +10
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15627?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/compare/54e71a74c81ecefb55bb35934b75f4cd1fb3ded1...37233aba17c3aae971a4f7f1eeac1b790a9229bc?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.25% <ø> (ø)` | :white_check_mark: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/54e71a74c81ecefb55bb35934b75f4cd1fb3ded1...37233aba17c3aae971a4f7f1eeac1b790a9229bc?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/54e71a74c81ecefb55bb35934b75f4cd1fb3ded1...37233aba17c3aae971a4f7f1eeac1b790a9229bc?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `91.76% <0%> (-0.35%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/54e71a74c81ecefb55bb35934b75f4cd1fb3ded1...37233aba17c3aae971a4f7f1eeac1b790a9229bc?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.87% <0%> (-0.1%)` | :x: |
| [pandas/tseries/index.py](https://codecov.io/gh/pandas-dev/pandas/compare/54e71a74c81ecefb55bb35934b75f4cd1fb3ded1...37233aba17c3aae971a4f7f1eeac1b790a9229bc?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvaW5kZXgucHk=) | `95.39% <0%> (+0.09%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15627?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15627?src=pr&el=footer). Last update [54e71a7...37233ab](https://codecov.io/gh/pandas-dev/pandas/compare/54e71a74c81ecefb55bb35934b75f4cd1fb3ded1...37233aba17c3aae971a4f7f1eeac1b790a9229bc?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). @DaanVanHauwermeiren Thanks!