Document patterns to build applications using Daml Finance

[matteolimberto-da]

We should document the patterns used in the daml-finance-app to work with Daml Finance components.

I am specifically referring to

• the factory pattern
• writing the UI against interfaces rather than concrete template implementations

This will be easier once interface views + subscriptions are part of the official Daml doc.

[bart-tydei]

I agree with "writing the UI against interfaces rather than concrete template implementations"

[matteolimberto-da]

An additional pattern that should be explained here is to implement the Lifecycle interface as a side-contract rather than directly in the instrument

[matteolimberto-da]

Client application architecture documented in https://github.com/digital-asset/daml-finance/pull/397

[matteolimberto-da]

Factory Pattern issue --> https://github.com/digital-asset/daml-finance/issues/850