Adding documentation for using multiple proxies

Created by: jbranchaud

I noticed in issue #1780 (closed) that there was discussion around support for declaring multiple proxies in development. It looks like this was implemented in #1790 and included as part of react-scripts 1.0.0 (#2172 (closed)).

I'd like to add documentation to the README for this feature, but I'm having trouble figuring out the exact semantics of it. I'm going to continue to investigate how it works. Any pointers in the right direction would be much appreciated.