DOC: Add documentation about cpplint
1) Minor spelling and command errors.
2) Add that we also use AppVeyor for testing.
3) With the introduction of `cpplint`, document it and how to use.

Follow-up to #14814.

@jreback , @jorisvandenbossche : There's a massive backlog on Travis with a good number of old builds (many of which are mine).  Do you think you could cancel them before they start? ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14890?src=pr) is 85.31% (diff: 100%)
> Merging [#14890](https://codecov.io/gh/pandas-dev/pandas/pull/14890?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14890   diff @@
==========================================
  Files           144        144          
  Lines         51006      51016    +10   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43512      43523    +11   
+ Misses         7494       7493     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [033d345...db09105](https://codecov.io/gh/pandas-dev/pandas/compare/033d34596f5327472007f0f15029b86050ee2592...db091058aa4d8f9a150e78153c0738d5c113c1fd?src=pr) thanks!