Document how to authtenticate to gcs when using google storage

[jkhelil]

Document how to authtenticate to gcs when using google storage

[jkhelil]

/kind documentation

[tekton-robot]

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

In response to this:

/kind doc

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[anithapriyanatarajan]

@jkhelil I would like to address this.

[anithapriyanatarajan]

@jkhelil PR submitted for this issue - #1322. Also could you suggest where to maintain automation scripts to run this tutorial