Document in FAQ whether/how SMTs affect message schemas [DBZ-457]

Migrated from [DBZ-457](https://issues.redhat.com/browse/DBZ-457)

I had been wondering the same and also was asked at Devoxx: if an SMT alters the type of a field (e.g. the [timestamp converter|https://github.com/debezium/debezium/pull/306]), will this reflect in the message's schema as given inline in case of JSON or externally using the schema registry?

I reckon it'll be transparent when applying the SMT on the sink side, but on the source side it's an interesting question.

Start by investigating how source-side and sink-side SMTs affect message schemas, including inline JSON schemas and external schema registry representations. Document the confirmed behavior and the timestamp-converter example in the project's FAQ; the issue names no documentation file or test to begin with.

Written by the indexing model from the issue text.