feat(aws-codepipeline): New Pipeline#addStage convenience method.

Added to make creating Stage objects a little more concise.

packages/@aws-cdk/aws-codepipeline/README.md

@@ -16,6 +16,13 @@ const sourceStage = new Stage(this, 'Source', {
 });
 ```
 
+There's also a utility method on the `Pipeline` class that can be used for this purpose:
+
+```ts
+// equivalent to the code above:
+const sourceStage = pipeline.addStage('Source');
+```
+
 Add an Action to a Stage:
 
 ```ts

packages/@aws-cdk/aws-codepipeline/lib/pipeline.ts

@@ -121,6 +121,19 @@ export class Pipeline extends cdk.Construct implements events.IEventRuleTarget {
         }));
     }
 
+    /**
+     * Convenience method for creating a new {@link Stage},
+     * and adding it to this Pipeline.
+     *
+     * @param name the name of the newly created Stage
+     * @returns the newly created Stage
+     */
+    public addStage(name: string): Stage {
+        return new Stage(this.parent!, name, {
+            pipeline: this,
+        });
+    }
+
     /**
      * Adds a statement to the pipeline role.
      */

packages/@aws-cdk/aws-codepipeline/test/integ.pipeline-code-deploy.ts

@@ -24,13 +24,13 @@ const pipeline = new codepipeline.Pipeline(stack, 'Pipeline', {
     artifactBucket: bucket,
 });
 
-const sourceStage = new codepipeline.Stage(stack, 'Source', { pipeline });
+const sourceStage = pipeline.addStage('Source');
 const sourceAction = bucket.addToPipeline(sourceStage, 'S3Source', {
     bucketKey: 'application.zip',
     artifactName: 'SourceOutput',
 });
 
-const deployStage = new codepipeline.Stage(stack, 'Deploy', { pipeline });
+const deployStage = pipeline.addStage('Deploy');
 new codedeploy.PipelineDeployAction(stack, 'CodeDeploy', {
     stage: deployStage,
     inputArtifact: sourceAction.artifact,