Document: Ema + Flat Data

[srid]

Flat Data is a suitable model to decouple external data (database, cloud data, etc.) from Ema's unionmount-based Dynamic style of programming.

Document how to do this, along with examples.

cf. faezs/astaghfirullah#1

[srid]

It doesn't have to be Github-specific as https://github.com/EmaApps/TheMotteDashboard demonstrates.