Document and facilitate t-shirt size deployments

In #29 and #32 we added the ability to configure the deployment of the elasticsearch VMs and O365 Plugins, There are also a number of other configuration options not currently exposed via the default `azuredeploy.parameters.json` file.

We need to document these options clearly and provide some example "t-shirt size" configurations to guide users. That is, what configuration should be used for "5000 students", "10000 students", "25000 students' etc.