Document new Channel CRDs Default Mechanism

[nachocano]

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

Introduced a new ConfigMap for setting up default Channel CRDs. We need to properly document it.

How will this impact our users?

v0.9 will remove all Channel provisioner-based stuff, so they will have to switch to Channel CRDs. Once they do that, they will need a way to configure defaults. When doing so, they will need to understand how to properly do that configuration, i.e., they need documentation.

As we are not removing anything in v0.8, they will not be immediately impacted.

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

v0.8

Context

Link to associated PRs or issues from other repos here.

1. Default Channel support for Channel CRDs eventing#1560
2. Support for default channel eventing#1343

Additional info

In 0.7 we've introduced the new Channel CRDs model. In 0.8 we have a new defaulting mechanism for these Channel CRDs. We should properly document it, with few examples.