Support plugin docs auto generation

[daisy-ycguo]

Support plugin docs auto generation.
My idea is to generate a README.md for plugin based on two inputs:

• some manually written part in md files, e.g. plugins/*/docs/header.md, and plugins/*/docs/footer.md
• the auto generated help doc by package github.com/spf13/cobra/doc

[daisy-ycguo]

/assign

[daisy-ycguo]

@maximilien FYI

[daisy-ycguo]

I create a golang util to generate README.md for a plugin which includes a generated Usage part in the middle of a header and a footer. The content of header and footer are reading from docs/header.md and docs/footer.md. @maximilien and I will use this util to generate README.md for eventing source plugins.

Since the repo client-contrib is not a go module. I'm not able to find a place to put this golang util which could be shared by all plugins. I have to put it to maximilien/kn-source-pkg/pkg/util, which is a private repo. I'm seeking advice to contribute this util to some place of Knative, maybe to knative/client/lib.