Doc: Added docstring to Groupby mean
Added examples of the usage and what is returned
Hello @cxl923cc! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on July 07, 2018 at 20:31 Hours UTC Thanks for the PR! There is some PEP 8 problem (basically formatting issues) which needed to be fixed. Also you may want to use `git diff upstream/master -u -- "*.py" | flake8 --diff` to check if there's PEP 8 problem before pushing. Also, you may want to  use "DOC: update the pandas.core.groupby.GroupBy.mean docstring" as the PR title so developers knows exactly which method/functions you are updating the docstring. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20910?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@dcbf8b5`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20910/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20910?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20910   +/-   ##
=========================================
  Coverage          ?   91.82%           
=========================================
  Files             ?      153           
  Lines             ?    49490           
  Branches          ?        0           
=========================================
  Hits              ?    45443           
  Misses            ?     4047           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <100%> (?)` | |
| #single | `41.85% <77.77%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20910?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/20910/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `92.66% <100%> (ø)` | |

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[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20910?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/20910?src=pr&el=footer). Last update [dcbf8b5...1eeedd7](https://codecov.io/gh/pandas-dev/pandas/pull/20910?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 This is my first pull request. Could someone please help to review the new commit I have made and let me know whether I should make any further change to the Docstring? thanks @cxl923cc and @mroeschke for the fixup!