Versioning Resources

Today, we don't always bump versions of resources when updating them.
The guideline has been to bump versions only when behavior changes,
but it's hard to figure out when the behavior has changed (a change
that could be trivial to one user could be meaningful to another).

Not bumping resource versions when changing them causes issues where
the resource definition becomes dependent on the time when it was applied
 by the user - which causes unexpected failures as described in
tektoncd#784.

This issue also came up as an issue where users cannot depend on the
Step indices because they can change:
tektoncd/community#572 (review).

In TEP-0003, we already proposed that a policy for versioning of resources:
https://github.com/tektoncd/community/blob/main/teps/0003-tekton-catalog-organization.md#versioning-resources

In Catalog Working Group on 01/13/2022, we revisted that policy and:
- agreed to follow the versioning policy
- make it easier to bump resources (see tektoncd/plumbing#994)

This change documents the versioning policy.

Fxes tektoncd#784

CONTRIBUTING.md

@@ -30,6 +30,12 @@ The process for contributing looks like this:
 
 ## How to Contribute a new version of a Task or Pipeline
 
+To provide stability of resources, we require that resources have to be bumped
+to new versions when making changes instead of modifying existing versions. This
+ensures that the resource definition is independent of the time it was applied to
+the cluster. Changes to existing versions may be permitted only for non-functional
+parts of the resources, e.g. labels and annotations, but we try to keep that minimal.
+
 If you are planning to add a new version of a Task or Pipeline make sure to
 separate your changes from the copied task. This makes it easy for reviewers to
 review the changes and not the actual copy.