Add browser support documentation.

@pfiller /cc @notdoug re: [recent ticket](https://iridesco.zendesk.com/agent/#/tickets/127591).

Adds an itty-bitty block to the README regarding browser support — this has come up twice in the last week or so (that referenced ticket as well as [harvestapp#4884](https://github.com/harvesthq/harvestapp/pull/4884#issuecomment-56822598)), so it’s about time we wrote it down.