Adding documentation for Event Registry

[samodell]

What is changing? (Please include as many details as possible.)

Adding Registry, which provide Event Consumers with a mechanism to discover what event types they can consume from the eventing mesh.

How will this impact our users?

Added functionality; needs documentation

In what release will this take happen (to the best of your knowledge)?

v0.6

Context

Link to associated PRs or issues from other repos here.

1. Event Registry eventing#929
2. https://github.com/nachocano/eventing/blob/b3115230d5770d481a0e22688490ac41aad13f1a/docs/registry/README.md

Additional info

I recommend adding this info as a new page in https://github.com/knative/docs/tree/master/docs/eventing , with a file name like using-eventing-registry.md or something similar.

Suggested title: Using the Eventing Registry