ENH: Series.append now has ignore_index kw
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

not sure if there is any reason we haven't supported...

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13677?src=pr) is **84.38%**

> Merging [#13677](https://codecov.io/gh/pydata/pandas/pull/13677?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13677   diff @@
==========================================
  Files           142        142          
  Lines         51223      51223          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43224      43224          
  Misses         7999       7999          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [6b9cd15...4bc7b54](https://codecov.io/gh/pydata/pandas/compare/6b9cd15f6a655b1ade2c571e32e142bf56dde769...4bc7b54fba5b51dc6521129698fe6110f867930d?src=pr)
 lgtm. just minor test comment.
 updated test, and now green.
 thanks!
