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: llvm/llvm-project#54965

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

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

Author: AaronBallman

llvm/docs/DeveloperPolicy.rst

@@ -180,6 +180,30 @@ etc) should be added to the ``llvm-test`` test suite.  The llvm-test suite is
 for coverage (correctness, performance, etc) testing, not feature or regression
 testing.
 
+Release Notes
+-------------
+
+Many projects in LLVM communicate important changes to users through release
+notes, typically found in ``docs/ReleaseNotes.rst`` for the project. Changes to
+a project that are user-facing, or that users may wish to know about, should be
+added to the project's release notes at the author's or code reviewer's
+discretion, preferably as part of the commit landing the changes. Examples of
+changes that would typically warrant adding a release note (this list is not
+exhaustive):
+
+* Adding, removing, or modifying command-line options.
+* Adding, removing, or regrouping a diagnostic.
+* Fixing a bug that potentially has significant user-facing impact (please link
+  to the issue fixed in the bug database).
+* Adding or removing optimizations that have widespread impact or enables new
+  programming paradigms.
+* Modifying a C stable API.
+* Notifying users about a potentially disruptive change expected to be made in
+  a future release, such as removal of a deprecated feature.
+
+Code reviewers are encouraged to request a release note if they think one is
+warranted when performing a code review.
+
 Quality
 -------