Update PR template to clarify approval requirement

[fxamacker]

Clarify and highlight that the proposal for the PR must be approved before it is opened.

REQUIRED: before opening a PR, comment on the related issue saying you plan to work on it, and wait for a maintainer to reply that you can proceed. Approval given to someone else, or approval of the idea in general, does not extend to you. No reply means not approved. PRs opened without this approval may be closed.