Add Go libraries compatibility policy

This change proposes that breaking changes may be introduced to the Go client libraries,
as long as existing yaml/json CRD definitions remain unaffected.

Breaking changes to the Go client libraries do not affect the Dashboard team, as the Dashboard
team no longer uses these libraries. The CLI team has decided that the impact of such changes
on them is acceptable.

api_compatibility_policy.md

@@ -85,3 +85,10 @@ These changes must be approved by [more than half of the project OWNERS](OWNERS)
 
 Tekton Pipelines [maintains a list of features that have been deprecated](https://github.com/tektoncd/pipeline/tree/main/docs/deprecations.md)
 which includes the earliest date each feature will be removed.
+
+## Go libraries compatibility policy
+
+Tekton Pipelines may introduce breaking changes to its Go client libraries, as long as these changes
+do not impact users' yaml/json CRD definitions. For example, a change that renames a field of a CRD
+would need to follow the policy on [backwards incompatible changes](#backwards-incompatible-changes),
+but a change that renames the Go struct type for that field is allowed.