docs(experimental-ec2-pattern): Add changeset

.changeset/happy-badgers-compare.md

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+---
+"@guardian/cdk": minor
+---
+
+feat(experimental-ec2-pattern): Pattern to deploy ASG updates w/CFN
+
+Included in this update is a new experimental pattern `GuEc2AppExperimental`, which can be used in place of a `GuEc2App`:
+
+```ts
+import { GuEc2AppExperimental } from "@guardian/cdk/lib/experimental/patterns/ec2-app";
+```
+
+This pattern will add an [`AutoScalingRollingUpdate` policy](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-attribute-updatepolicy.html#cfn-attributes-updatepolicy-rollingupdate)
+to the autoscaling group.
+This allows application updates to be performed like a standard CloudFormation update,
+and using the custom logic provided by Riff-Raff's `autoscaling` deployment type is unnecessary.
+
+This experimental pattern has few requirements.
+
+## Add the build number to the application artifact
+This change requires versioned artifacts.
+
+The easiest way to achieve this is by adding the build number to the filename of the artifact:
+
+```ts
+import { UserData } from "aws-cdk-lib/aws-ec2";
+// Use a GitHub Actions provided environment variable
+const buildNumber = process.env.GITHUB_RUN_NUMBER ?? "DEV";
+
+const userData = UserData.forLinux();
+userData.addCommands(`aws s3 cp s3://dist-bucket/path/to/artifact-${buildNumber}.deb /tmp/artifact.deb`);
+userData.addCommands(`dpkg -i /tmp/artifact.dep`);
+```
+
+## `riff-raff.yaml`
+The `riff-raff.yaml` file should remove the `deploy` action of the `autoscaling` deployment type.
+Though including it shouldn't break anything, it would result in a longer deployment time as instance will be rotated by both CloudFormation and Riff-Raff's custom logic.
+
+The `uploadArtifacts` step of the `autoscaling` deployment type should still be included, with the `cloud-formation` deployment type depending on it.
+This step uploads the versioned artifact to S3.
+
+> [!TIP]
+> An [auto-generated `riff-raff.yaml` file](https://github.com/guardian/cdk/blob/main/src/riff-raff-yaml-file/README.md) meets this requirement.