Add template for issue and PR

[atoato88]

This PR add template files for issue and PR on GitHub.
It's good to clarify information about issue and PR.

I referred k/k's templates for issue and PR.
But that is too much input items for this repository I think, I cut some items from it for ease to input.

[atoato88]

/assign @justinsb

[justinsb]

It's probably a lot of information for a library (vs something directly user facing), but I ❤️ the idea and we can always trim it down later if we need to!

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: atoato88, justinsb

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[justinsb]

/retest

Test failure was unrelated, should have been fixed by #155

[atoato88]

@justinsb
Thank you for approving.

... we can always trim it down later if we need to!

Yes, I agree with your idea. We can always fit the template to our usecase.