Add information on bodyMode to the README

[ycz]

I've had issues with binding of tooltips to dynamic content in one of my projects for a while now, and was using the method suggested in the README of calling ReactTooltip.rebuild() whenever the content changed, which was causing performance issues. Someone I was working with found out about the existence of bodyMode by digging through the codebase, and after trying it out I found that it basically solved my problem in one fell swoop.

It'd be great if the README included information on what this option does, its ideal use cases, and why it isn't the default mode, but at the very least I think it ought to be listed under the options and mentioned in the "Tooltip not binding to dynamic content" section.

Thanks!