string format for height causes page to blow up

[jacobworrel]

first mentioned here: #31

I've found that if I use a height format other than a pure number, such as "100px", it breaks so hard I have to force kill my Chrome tab. Easy enough to fix, but it might be worth noting in the documentation that heights must be provided in only that format.

[jacobworrel]

Added a comment to the docs - it still might be a good idea to support strings like 100px although I'm not sure if this is an issue with react-windowed-select or react-select so would need to dig deeper.