update pull request template

Created by: zryfish

What type of PR is this?

/kind documentation

What this PR does / why we need it:

We are adopting more features of Prow, need to align PULL_REQUEST_TEMPLATE with Kubernetes. It's more friendly. release-note section is required after, every PR author is responsible for writing readable release note if it introduces user facing changes. Otherwise, just write None

Which issue(s) this PR fixes:

Fixes #

Special notes for reviewers:

None

Does this PR introduced a user-facing change?

None