Add the doc for Approval (#176)

* Change the lock image.

* Add the doc for approval

Author: Noah Hanjun Lee

docs/concepts/approval.md

@@ -0,0 +1,9 @@
+# Approval
+
+Request approvals to commiters who has commited for the deployment. The reviewers must have at least read access to the repository. And you can configure how many reviewers needs to approve.
+
+For more information, you can check the [document](./deploy.yml.md).
+
+Figure) Request approval
+
+![Approval](../images/approval.png)

docs/images/approval.png

@@ -15,6 +15,7 @@ nav:
     - "How it works": concepts/how-it-work.md
     - "deploy.yml": concepts/deploy.yml.md
     - Deployment: concepts/deployment.md
+    - Approval: concepts/approval.md
     - Lock: concepts/lock.md
     - Permission: concepts/permission.md
     - Chatops: concepts/chatops.md