Query time lookups need a tutorial

[fjy]

Query time lookups need a tutorial of the problems they solve and how they can be used. It is also worthwhile to add an entry in the TOC under "Operations" about QTL, described in a way that folks facing the problem can find and understand.

[drcrallen]

This is blocked by #1576

[b-slim]

This is blocked by the implementation of a lookup manager as a druid component not as an extension.
First milestone will be the following PR [https://github.com//pull/2291]

[rasahner]

I think it's not necessary, or even desirable, to wait until the implementation is complete before having user-facing documentation, at least in draft form. It's a useful way to state what problem is being solved (what the requirements are). It can also suggest integration tests.

[drcrallen]

@rasahner I think what you're describing is a proposal.The User docs are a major pain to keep in sync with the development docs as code reviews change things.