Update README with Firefox policy so it's clear to contributors in the future

[wistcc]

Here some things to take on count when updating the README:

• How to run the developer mode fo Firefox
• Important notes about Firefox policy

[martynchamberlin]

This is also a helpful tidbit: http://stackoverflow.com/a/31813035/1591507

It's helpful running chrome.storage.local.clear() when you want to experience the extension as a first time user.