DOC: Added note to io.rst regarding reading in mixed dtypes
- [x] closes #13746 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

This is my first time contributing to the docs, I will gladly take any criticism.

Thanks

@jreback Updated to ipython blocks. I accidentally amended my commit instead of rebasing, but it looks like it doesn't really matter.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13782?src=pr) is 85.23% (diff: 100%)

> Merging [#13782](https://codecov.io/gh/pydata/pandas/pull/13782?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13782   diff @@
==========================================
  Files           140        140          
  Lines         50415      50415          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42971      42971          
  Misses         7444       7444          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [a3cddfa...7400607](https://codecov.io/gh/pydata/pandas/compare/a3cddfa9c4302c13747509caef7a056159010adf...7400607fa5d9ccef19a826d4581c65d393b4e237?src=pr)
 > I noticed that there are two Travis CI builds for your commits, so you might want to rebase next time if you can

I don't think that does matter, if you rebase (or amend, which is in effect quite the same: you change the commit) and force push again, you will also get an additional travis build.

>  accidentally amended my commit instead of rebasing, but it looks like it doesn't really matter.

That does indeed not matter. Amending is perfectly fine (making a separate commit and pushing that is also fine), rebasing is only strictly necessary if there are merge conflicts to solve (or to incorporate changes from master, but that is not essential here)
 @wcwagner I added some comments, but it is a really nice addition to the docs!
 @jorisvandenbossche Thanks for the help!
 @wcwagner looks really good. just some more links and think will be good.

I would also add a note in whatsnew (maybe enhancements) with a link to the new documentation (eg saying that its updated).
 thanks @wcwagner 
great PR!

check out http://pandas-docs.github.io/pandas-docs-travis/ in a while to see the built docs. If things don't look exactly right, pls issue a followup PR.
