Update the developer policy to mention release notes

As a project, Clang has gotten negative public feedback about our lack
of communicating changes to users. There are comments on places like
Hacker News or Reddit where users have (rightfully) been confused as to
what changes happen in a given release, leading to misinformation like
Clang not adding support for C++20 features:
https://news.ycombinator.com/item?id=28761464.

This documents the expectation that changes which impact users should
have release notes, and it's normal for code reviewers to ask an author
to add a release note for a given change.

This addresses: https://github.com/llvm/llvm-project/issues/54965

RFC: https://discourse.llvm.org/t/rfc-update-developer-policy-on-release-notes/61856

Differential Revision: https://reviews.llvm.org/D123957