Add link to Katakoda sample to community samples.

[evankanderson]

Adds a link to the interactive Katakoda sample created by Swapnil Bawaskar
(@sbawaska) to the community samples. This lets people try out Knative without
needing to install anything on their local machine.

[evankanderson]

/assign @sbawaska

[knative-prow-robot]

@evankanderson: GitHub didn't allow me to assign the following users: sbawaska.

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In response to this:

/assign @sbawaska

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[vaikas]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: evankanderson, vaikas

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