Add the documentation for simple-pbt

[tenzen-y]

Signed-off-by: tenzen-y yuki.iwai.tz@gmail.com

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

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[johnugeorge]

Can you rebase

@tenzen-y

[tenzen-y]

Can you rebase

@tenzen-y

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