Since we have recently changed the name of this module to 'cloudevents', users
may not be aware that version 2.0.2 is out of date, since it won't automatically
be upgraded on `npm install`.

This commit adds a postinstall script to warn users of this fact, and adds a
bold, top-level warning to the README.

Fixes: cloudevents#291

Signed-off-by: Lance Ball <lball@redhat.com>

squash