chore: Update CONTRIBUTING.md with authorization instructions

CONTRIBUTING.md

@@ -38,8 +38,6 @@ GITPLOY_GITHUB_CLIENT_SECRET=XXXXXXXXXXXXX
 GITPLOY_STORE_SOURCE=file:./sqlite3.db?cache=shared&_fk=1
 ```
 
-Note that if you want to interact with GitHub in the local environment, you should install tunneling tools such as [ngork](https://ngrok.com/) and expose your local server.
-
 3\. Run the server:
 
 ```
@@ -71,3 +69,22 @@ REACT_APP_GITPLOY_SERVER=http://localhost
 ```
 npm start
 ```
+
+### Authorization
+
+1\. Run with ngrok
+
+Connect to the public host via [ngrok](https://ngrok.com/) to authorize with GitHub OAuth. 
+
+```shell
+ngrok http 80
+```
+
+2\. Configure GitHub OAuth Apps
+
+Configure the Homepage URL and Authorization callback URLs with the public host which generated by ngrok.
+
+3\. Access the index page
+
+Access the index page of server with the browser. You can find the user is created after authorization.
+