[DOCS] Add example of how to preserve order of columns with usecols.
#### What's this PR do?
Adding an example of how to preserve order when using the ``usecols`` parameter.  I've noticed that some folks are still unaware that the columns will not necessarily be returned in the specified order passed to ``usecols``.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19746?src=pr&el=h1) Report
> Merging [#19746](https://codecov.io/gh/pandas-dev/pandas/pull/19746?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/192bd46fd89189b7905d87db95842de6299285d8?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19746/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19746?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19746      +/-   ##
==========================================
+ Coverage   91.58%    91.6%   +0.02%     
==========================================
  Files         150      150              
  Lines       48862    48864       +2     
==========================================
+ Hits        44750    44763      +13     
+ Misses       4112     4101      -11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19746?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/19746/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.32% <ø> (ø)` | :arrow_up: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19746/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.45% <0%> (-0.39%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19746/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.45% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/19746/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `94.9% <0%> (+0.01%)` | :arrow_up: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/19746/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.31% <0%> (+0.03%)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19746/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.22% <0%> (+0.06%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19746/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19746?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/19746?src=pr&el=footer). Last update [192bd46...68efc4b](https://codecov.io/gh/pandas-dev/pandas/pull/19746?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @EricChea! Thanks for updating the PR.



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

##### Comment last updated on February 18, 2018 at 18:27 Hours UTC ping @jreback @jorisvandenbossche : this is green and ready to merge Thanks @EricChea! Thanks for reviewing  @TomAugspurger 