* Rewrite "adding React to existing app"
Fixes#988
* Some copy
* typo
* Update babel instructions
* Update umd link
* Add prod minification section
* Show "button" example in several targets
* wip
* More
* More
* tweak
* yas
* Multi root tip
* moaar
* alot
* Tweak links
* Explain better
* better lead
* tweaks
* tweaks
* wording
* More reassuring tone
* Grammar
* wording
* feedback from readers
* Use id, not class
* More nits
* Re-add a useful section
Dan Abramov