DOC: update brief documentation on visualization projects
- [ X] passes `git diff upstream/master | flake8 --diff`

Minor Documentaion change to note that Vincent project has intentionally been abandoned and is not longer maintained.  Add a pointer to IPython Vega successor.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13528?src=pr) is **84.33%**

> Merging [#13528](https://codecov.io/gh/pydata/pandas/pull/13528?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13528   diff @@
==========================================
  Files           138        138          
  Lines         51107      51107          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43103      43103          
  Misses         8004       8004          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [9e73c71...b1fbee0](https://codecov.io/gh/pydata/pandas/compare/9e73c714433fe071a4441bf1e22dd6c01bd01717...b1fbee0fb810239f0c4d03ae10aef231e0a1d13d)
 Thanks!
