Add the fifth assignment

[refs #6acefea3d1d5]

Author: jfahrer

developing/examples/webapp/assignment5.md

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+## Integrate your load balancer
+* Add a new service definition
+  * Call it `lb`
+  * Use your load balancer image
+    * Or use jfahrer/lb:v3
+  * Make sure to assign the PROXY_UPSTREAM environment variable
+    * Needs to reference the correct service and port
+  * Publish port 80
+* Start your services
+* Use your browser to verify it works
+  * http://localhost
+* Scale your web app
+  * Stop publishing ports for your web app