Write documentation on Vendorless Knative

[cardil]

The introduction of vendorless in the Knative projects requires some documentation, for contributors, so the knowledge can be shared asynchronously.

/kind doc

[knative-prow]

@cardil: The label(s) kind/doc cannot be applied, because the repository doesn't have them.

In response to this:

The introduction of vendorless in the Knative projects requires some documentation, for contributors, so the knowledge can be shared asynchronously.

/kind doc

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