Update README.md to explicitly mention this polyfill is meant to act as a no-op on Safari as it does on Firefox

[rpl]

Given that we got some issues filed related to Safari (e.g #234 and #299) it may be good to update README.md#supported-browsers to explicitly mention that:

• Safari does provide its own natively implemented Promise-based extensions APIs
• and this polyfill is meant to act as a no-op on Safari (as it does on Firefox).

It may also be reasonable to mention in README.md#known-limitations-and-incompatibilities some known incompatibilities (e.g. I recall that browser.runtime.onMessage.addListener may not be working as in the Firefox promised API when a listener does return a promises, but we'll need to double-check that before mentioning it explicilty)