Create README.md for Hello World App

We have a very simple App called Hello World App that is version-controlled in bitbucket. It's meant to illustrate how a developer can add a new menu item to IGB.

Repository link: https://bitbucket.org/lorainelab/hello-world-igb-app/src/master/

However, it lacks a README.md file. We should create one that summarizes the purpose and functionality of App.

We need explain to the user/developer where the App will appear once it's added, i.e., which menu it gets added to.

Note that the README.md will be base-64 encoded and included in the OBR index file created for the App. Search Jira for details.