Mention OnDemand in MOBILECONFIG.md

[slokhorst]

This adds a brief explainer of the OnDemandEnabled and OnDemandRules keys that can be set in a MobileConfig.

For now, I added a link to https://gist.github.com/deg0nz/bec056213aef57d84b05b21bb046a16c, which has an example for OnDemandRules. Since it can become quite complex, I'm not sure if we should include it in this document, or maybe split it out in a separate file.