docs: Add changeset

Author: akash1810

.changeset/silent-planets-invent.md

@@ -0,0 +1,70 @@
+---
+"@guardian/cdk": patch
+---
+
+Correctly validate stack existence when instantiating a `RiffRaffYamlFile` to generate `riff-raff.yaml`.
+
+Given the below set of stacks, an `Error` will now be thrown as there is a missing instance of `MyDatabaseStack` for `CODE`.
+
+```typescript
+new MyApplicationStack(app, "App-CODE-deploy", {
+  env: {
+    region: "eu-west-1",
+  },
+  stack: "deploy",
+  stage: "CODE",
+});
+new MyApplicationStack(app, "App-PROD-deploy", {
+  env: {
+    region: "eu-west-1",
+  },
+  stack: "deploy",
+  stage: "PROD",
+});
+
+new MyDatabaseStack(app, "Database-PROD-deploy", {
+  env: {
+    region: "eu-west-1",
+  },
+  stack: "deploy",
+  stage: "PROD",
+});
+```
+
+Previously, the validation would incorrectly pass.
+
+<details><summary>Invalid `riff-raff.yaml`</summary>
+<p>
+
+```yaml
+allowedStages:
+  - CODE
+  - PROD
+deployments:
+  cfn-eu-west-1-deploy-my-application-stack:
+    type: cloud-formation
+    regions:
+      - eu-west-1
+    stacks:
+      - deploy
+    app: my-application-stack
+    contentDirectory: /private/var/folders/0_/pvjwppsx5cl19t4n6_rmm_y80000gp/T/cdk.out9xIUJu
+    parameters:
+      templateStagePaths:
+        CODE: App-CODE-deploy.template.json
+        PROD: App-PROD-deploy.template.json
+  cfn-eu-west-1-deploy-my-database-stack:
+    type: cloud-formation
+    regions:
+      - eu-west-1
+    stacks:
+      - deploy
+    app: my-database-stack
+    contentDirectory: /private/var/folders/0_/pvjwppsx5cl19t4n6_rmm_y80000gp/T/cdk.out9xIUJu
+    parameters:
+      templateStagePaths:
+        PROD: Database-PROD-deploy.template.json
+```
+
+</p>
+</details>