chore: update contribution guide with feature flag support for v2 (aw…

…s#13084)

Update how feature flags that are destined to be dropped in v2 should be
developed.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

CONTRIBUTING.md

@@ -849,6 +849,22 @@ CDK](https://github.com/aws/aws-cdk/issues/3398) we will either remove the
 legacy behavior or flip the logic for all these features and then
 reset the `FEATURE_FLAGS` map for the next cycle.
 
+#### CDKv2
+
+We have started working on the next version of the CDK, specifically CDKv2. This is currently being maintained
+on a separate branch `v2-main` whereas `master` continues to track versions `1.x`.
+
+Feature flags introduced in the CDK 1.x and removed in 2.x, must be added to the `FUTURE_FLAGS_EXPIRED` list in
+[cx-api/lib/features.ts](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/cx-api/lib/features.ts)
+on the `v2-main` branch.
+This will make the default behaviour in CDKv2 as if the flag is enabled and also prevents users from disabling
+the feature flag.
+
+A couple of [jest helper methods] are available for use with unit tests. These help run unit tests that test
+behaviour when flags are enabled or disabled in the two major versions.
+
+[jest helper methods]: https://github.com/aws/aws-cdk/blob/master/tools/cdk-build-tools/lib/feature-flag.ts
+
 ### Versioning and Release
 
 The `release.json` file at the root of the repo determines which release line

packages/@aws-cdk/cx-api/test/features.test.ts

@@ -1,7 +1,12 @@
+import { testLegacyBehavior } from 'cdk-build-tools/lib/feature-flag';
 import * as feats from '../lib/features';
 
 test('all future flags have defaults configured', () => {
   Object.keys(feats.FUTURE_FLAGS).forEach(flag => {
     expect(typeof(feats.futureFlagDefault(flag))).toEqual('boolean');
   });
+});
+
+testLegacyBehavior('FUTURE_FLAGS_EXPIRED must be empty in CDKv1', Object, () => {
+  expect(feats.FUTURE_FLAGS_EXPIRED.length).toEqual(0);
 });