Add rfc for script run stage

docs/rfcs/0011-script-run-stage.md

@@ -0,0 +1,107 @@
+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- Target Version: (1.x / 2.x)
+
+# Summary
+
+This RFC introduces a new way to enable users to use "script run stages” that users can execute any commands in their pipelines.
+
+# Motivation
+
+Currently, users can use only stages that PipeCD has already defined. However some users want to define new stages by their use-cases as below. 
+
+- Deploying infrastructure by tools other than that PipeCD supports (terraform and kubernetes) such as SAM, cloud formation….
+- Running End to End tests
+- Interacting with external systems
+- Performing database migrations
+- notifying the deployed result
+
+`CUSTOM_SYNC` is implemented for the above use-cases, but it is for sync. 
+So more simply, some users want to execute commands.
+
+# Detailed design
+
+## feature
+
+1. execute any commands in their pipeline.
+
+```
+apiVersion: pipecd.dev/v1beta1
+kind: LambdaApp
+spec:
+  encryptedSecrets:
+    password: encrypted-secrets
+  pipeline:
+    stages:
+      - name: SCRIPT_RUN
+        env:
+          AWS_PROFILE: default
+        runs:
+          - "echo {{ .encryptedSecrets.password }} | sudo -S su"
+          - "sam build"
+          - "sam deploy -g --profile $AWS_PROFILE"
+```
+
+2. combine with other stage
+
+Compared to CUSTOM_SYNC, this stage can be combined with other stage.
+For example, 
+
+```
+apiVersion: pipecd.dev/v1beta1
+kind: KubernetesApp
+spec:
+  pipeline:
+    stages:
+      - name: K8S_CANARY_ROLLOUT
+        with:
+          replicas: 10%
+      - name: WAIT_APPROVAL
+        with:
+          timeout: 30m
+      - name: K8S_PRIMARY_ROLLOUT
+      - name: K8S_CANARY_CLEAN
+      - name: SCRIPT_RUN
+        env:
+          SLACK_WEBHOOK_URL: ""
+        runs:
+          - "curl -X POST -H 'Content-type: application/json' --data '{"text":"successfully deployed!!"}' $SLACK_WEBHOOK_URL"
+```
+
+3. execute any commands when rollback
+
+Users can define commands to execute when rolling back with `runsOnRollback`.
+If `runsOnRollback` is not set, nothing to execute when rolling back.
+
+```
+apiVersion: pipecd.dev/v1beta1
+kind: KubernetesApp
+spec:
+  pipeline:
+    stages:
+      - name: K8S_CANARY_ROLLOUT
+        with:
+          replicas: 10%
+      - name: WAIT_APPROVAL
+        with:
+          timeout: 30m
+      - name: K8S_PRIMARY_ROLLOUT
+      - name: K8S_CANARY_CLEAN
+      - name: SCRIPT_RUN
+        env:
+          SLACK_WEBHOOK_URL: ""
+        runs:
+          - "curl -X POST -H 'Content-type: application/json' --data '{"text":"successfully deployed!!"}' $SLACK_WEBHOOK_URL"
+        runsOnRollback:
+          - "curl -X POST -H 'Content-type: application/json' --data '{"text":"failed to deploy: rollback"}' $SLACK_WEBHOOK_URL"
+```
+
+# Alternatives
+
+**What's the difference between "CUSTOM_SYNC" and "SCRIPT_SYNC"?**
+
+"CUSTOM_SYNC" is one of the stages to **sync**, but "SCRIPT_RUN" is the stage to **execute commands**.
+
+# Unresolved questions
+
+It might be better to add `runsOnRollback` on each sync stage if users want to control when rollback.
+This is a just idea.