Description

Adding some basic blurb to the SDK's README to highlight our support for push notifications in Sandbox environments.

Check List

[ ] Are you changing anything with the public API?
[ ] Have you tested this change on real device?
[ ] Are your changes backwards compatible with previous SDK Versions?
[ ] Have you added unit test coverage for your changes?
[ ] Have you verified that your changes are compatible with all the operating system version this SDK currently supports?