chore: ABCI++ Integration docs

[samricotta]

Following the hackathon we need to have better documentation for people using ABCI++ with the usage of handlers etc. The feedback given was that a few people spent hours trying to find docs on enabling vote extensions although its mentioned in UPGRADING.md. Personally I think we need a guide or at least some more documentation on this. Although im sure this will be a working process, it would be great to aim to get something out for 0.50 release.

[alexanderbez]

Yes, this will be extremely helpful. The doc should describe what ABCI++ is, how to integrate it and how to enable it, along with a simple FAQ.