Documentation about tkn plugins is missing

[chmouel]

There is no documentation in README at least about the new plugin system of tkn

cc @vdemeester

/kind documentation

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
Stale issues rot after an additional 30d of inactivity and eventually close.
If this issue is safe to close now please do so with /close with a justification.
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/lifecycle stale

Send feedback to tektoncd/plumbing.

[vdemeester]

/lifecycle frozen

[piyush-garg]

assign @Senjuti256

[piyush-garg]

/assign @Senjuti256

[tekton-robot]

@piyush-garg: GitHub didn't allow me to assign the following users: Senjuti256.

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In response to this:

/assign @Senjuti256

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[Senjuti256]

/assign

[piyush-garg]

done with #2255