Severity

Add severity & payloadVersion to notifier docs

Related Pull Requests:
https://github.com/bugsnag/bugsnag-website/pull/189
https://github.com/bugsnag/bugsnag-event-worker/pull/59
https://github.com/bugsnag/bugsnag-ruby/pull/96
https://github.com/bugsnag/bugsnag-php/pull/39
https://github.com/bugsnag/bugsnag-cocoa/pull/14
https://github.com/bugsnag/bugsnag-java/pull/13
https://github.com/bugsnag/bugsnag-android/pull/15
https://github.com/bugsnag/bugsnag-js/pull/41
https://github.com/bugsnag/bugsnag-python/pull/23
https://github.com/bugsnag/bugsnag-node/pull/21