Add the documentation for simple-pbt

[tenzen-y]

Signed-off-by: tenzen-y [email protected]

What this PR does / why we need it:
As discussed in this, I added the documentation for the simple-pbt example.

cc: @a9p @gaocegege

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Checklist:

• Docs included if any changes are user facing

[coveralls]

Coverage remained the same at 73.431% when pulling 22951d7 on tenzen-y:add-document-for-simple-pbt into 4299856 on kubeflow:master.

[a9p]

Thanks for following up on the previous issue and adding these changes!

[johnugeorge]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: a9p, johnugeorge, tenzen-y

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [johnugeorge]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[johnugeorge]

Can you rebase

@tenzen-y

[tenzen-y]

Can you rebase

@tenzen-y

@johnugeorge
hmm. This PR has been merged into the master branch...