add release notes/changelog requirement to PR Template

Summary:
grabbou has to write release notes for every release, and that's getting ridonc. Have you seen the changelog from 0.48?

PR writers need to do their own, hopefully in a standardized format so it can be scripted.

Just a PR template change, just proofreading needed. Please mention if have missed or added any extra Categories, Types, or Where columns.

Here's a sample:

[GENERAL] [ENHANCEMENT] [PULL_REQUEST_TEMPLATE.md] - added release notes/changelog requirement to PR Template
Closes https://github.com/facebook/react-native/pull/15874

Differential Revision: D6054557

Pulled By: hramos

fbshipit-source-id: 5741b98a0efdb1a6aaaeedaa292403b82ce713f6