Create blogging guidelines for Knative.dev

[macruzbar]

Describe the change you'd like to see
Have a documented process for Knative.dev blog that encourages community members to contribute content to the Knative blog.

Additional context
This was discussed at the Documentation WG meeting of May 19th.

/assign @macruzbar

[macruzbar]

PR waiting to be merged here. Blogging guidelines done for now.

/close

[knative-prow-robot]

@macruzbar: Closing this issue.

In response to this:

PR waiting to be merged here. Blogging guidelines done for now.

/close

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