Documentation improvements for cloud event support

[dsyer]

1. The existing cloud event sample is neat, but there's no actual sign that it is using cloud events. We could fix that just by including the HTTP response in the README (see sample below).

2. It is easy to use the functionRouter to dispatch events based on the cloud event type (see sample below), so that would make a good example too for the main docs or the sample README

3. It's not clear from the docs on functionRouter that you need to make it the default function for it to work. E.g. by setting spring.cloud.function.definition=functionRouter.

Working sample: https://github.com/scratches/function-router.