Update README.md

Author: jssnull

README.md

@@ -1,8 +1,29 @@
 ## A Jenkins image for building and testing a Dockerized flask app
 
 Usage:
+Before building this image you have to create 
+a directory called: "secrets" and it should contain two files
+called jenkins-user and jenkins-pass, this files are used for storing temporaly jenkins initial user credentials.
+
+Example:
+```bash
+➜  flask-jenkins git:(master) ✗ tree secrets            
+secrets
+├── jenkins-pass
+└── jenkins-user
+
+0 directories, 2 files
+➜  flask-jenkins git:(master) ✗ cat secrets/jenkins-user
+admin
+➜  flask-jenkins git:(master) ✗ cat secrets/jenkins-pass
+admin
+➜  flask-jenkins git:(master) ✗ 
+```
+
+
+Having secret folder ready, run:
 ```bash
 docker-compose up
 ```
 It will run a jenkins master instance and will bind it 
-to port 8080 for web access and 50000 for jenkins slave communication.
+to port 8080 for web access and 50000 for jenkins slave communication.