Add handling breaking changes guide

docs/developer/breaking-changes.md

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+# Handling Breaking Changes
+
+## Before you begin
+
+* Make sure you've read [the backwards-compatibility document][backwards-compatibility-doc].
+* Write down in what way your change will break users. Think of cases where the
+  upgrade of Alloy to a new version would require a manual step beyond just
+  replacing the binary to keep all the functionality working.
+
+## Considerations
+
+Use the list of considerations below to generate ideas / approaches on how the
+change could be handled:
+
+* **Can we make this change non-breaking in Alloy code?**
+
+  Sometimes it's possible to make the change non-breaking with some extra
+  effort. For example:
+
+    * If a metric that is used on our official dashboard was renamed, we can add
+      an OR to a query and the dashboard can support both: new and old metric.
+    * If a CLI flag or a config option was renamed, we can add an alias that
+      allows us to use both names.
+
+  **NOTE:** If you choose this option, we may still want to make the breaking
+  change in the next major
+  release. [Make sure that you correctly track it][tracking-breaking-changes].
+
+* **Is this change covered by our backwards-compatibility RFC guarantees?**
+    * **Is this change out-of-scope of our guarantees?**
+        * For example: metrics that are not used on our official dashboards.
+    * **Is this change called out as an exception?**
+        * For example: non-stable functionality or breaking changes in the
+          upstream.
+
+  **NOTE:** If the change is not a breaking change by our definition, but it
+  would still cause breakage and/or frustration with our users, we may still
+  communicate it to the users. See the [communicating the breaking changes section][communicating].
+
+* **Can this change wait for the next major release?**
+
+  Sometimes changes are urgent, e.g. security fixes or users blocked on them,
+  but sometimes they can wait for the next major release. Also, consider how far
+  in the future is the next major release.
+
+  **NOTE:** If you do pick this option, make sure you
+  correctly [track the work necessary for the next major release][tracking-breaking-changes].
+
+* **Can we fork / update an existing fork to make this change non-breaking?**
+
+  Sometimes it may be preferable to bring back the old behaviour or handle the
+  breaking change in a backwards-compatible way in the upstream library.
+  Consider this as an option.
+
+  **NOTE:** If you choose this option, we may still want to make the breaking
+  change in the next major
+  release. [Make sure that you correctly track it][tracking-breaking-changes].
+
+* **Should we first deprecate / give a warning and make the breaking change
+  later?**
+
+  It may be a good idea for certain breaking changes to give a heads-up / add a
+  warning in one release and make the breaking change later. Consider this as an
+  option.
+
+* **Do we have an idea how widely the impacted feature is used and how many
+  users will be impacted?**
+
+  Sometimes this can help decide which approach to take.
+
+* **Do we need to discuss this with maintainers / a wider community?**
+
+  If there is no exception, no way to make the change non-breaking and we cannot
+  wait for the next major release, we may need to discuss this further and
+  consider options not listed here.
+
+## Decide the best approach
+
+We'd typically prefer options in the following order:
+
+1. Not make a breaking change at all - via code change in Alloy or in a fork or
+   upstream
+2. If not possible, we'd prefer to wait for the next major release
+3. If not possible, we'd consider using our backwards-compatibility scope
+   definition / exceptions / out-of-scope options
+4. If not possible, we'd likely need a wider discussion about this change and
+   communicate with users
+
+## Communicating the breaking changes
+
+Currently, we use our CHANGELOG.md to communicate the presence of breaking
+changes. They are also included on a GitHub release page under the notable
+changes section. For changes that require manual steps to migrate, we must also
+include a migration guide.
+
+## Tracking work that needs to be done for the next major release
+
+If we need to plan some work for the next major release, it's essential to track
+it correctly, so it's not lost, and we don't miss the rare opportunity to make
+some breaking changes.
+
+We currently use GitHub issues assigned to 2.0 milestone to track the issues
+planned for the next release.
+
+[tracking-breaking-changes]: #tracking-work-that-needs-to-be-done-for-the-next-major-release
+[backwards-compatibility-doc]: TODO
+[communicating]: #communicating-the-breaking-changes