Add docs for SCRIPT_RUN stage

docs/content/en/docs-dev/user-guide/managing-application/customizing-deployment/script-run.md

@@ -0,0 +1,64 @@
+---
+title: "Script Run stage"
+linkTitle: "Script Run stage"
+weight: 4
+description: >
+   Specific guide for configuring Script Run stage
+---
+
+`SCRIPT_RUN` stage is one stage in the pipeline and you can execute any commands.
+
+> Note: This feature is at the alpha status and only available for application of kind AWS Lambda.
+
+## How to configure SCRIPT_RUN stage
+
+Add a `SCRIPT_RUN` to your pipeline and write commands. 
+The commands run in the directory where this application configuration file exists.
+
+```yaml
+apiVersion: pipecd.dev/v1beta1
+kind: KubernetesApp
+spec:
+  name: canary-with-script-run
+  labels:
+    env: example
+    team: product
+  pipeline:
+    stages:
+      - name: K8S_CANARY_ROLLOUT
+        with:
+          replicas: 10%
+      - name: WAIT
+        with:
+          duration: 10s
+      - name: SCRIPT_RUN
+        with:
+          env:
+            MSG: "execute script1"
+          run: |
+            echo $MSG
+            sleep 10
+      - name: K8S_PRIMARY_ROLLOUT
+      - name: K8S_CANARY_CLEAN
+      - name: SCRIPT_RUN
+        with:
+          env:
+            MSG: "execute script2"
+          run: |
+            echo $MSG
+            sleep 10
+```
+
+![](/images/custom-sync.png)
+
+Note:
+1. You can use `SCRIPT_RUN` stage with any current supporting application kind.
+
+
+2. The public piped image available in PipeCD main repo (ref: [Dockerfile](https://github.com/pipe-cd/pipecd/blob/master/cmd/piped/Dockerfile)) is based on [alpine](https://hub.docker.com/_/alpine/) and only has a few UNIX command available (ref: [piped-base Dockerfile](https://github.com/pipe-cd/pipecd/blob/master/tool/piped-base/Dockerfile)). If you want to use your commands, you can:
+
+- Prepare your own environment container image then add [piped binary](https://github.com/pipe-cd/pipecd/releases) to it.
+- Build your own container image based on `ghcr.io/pipe-cd/piped` image.
+- Manually update your running piped container (not recommended).
+
+