Add Handler+Service tutorial to documentation.md

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Atm, Todo.fs and Aggregate.fs are the only real examples (aside from the samples/ folder) of what one might do to connect the basic domain logic to a running app.

Documenting the sort of things you put into the Handler and/or Service and how to balance that is a key piece of documentation that's been pointed out as missing....

Patterns to be covered:

• Command with no response with straight call in Service (todo does this)
• Command with decision result from Interpret function feeding out (don't think that's covered either here or in the Equinox /samples folder)
• Command with projection from final folded result post command application (todo does this)
• Query (see todo and aggregate)

General advice on what to do that the tutorial should cover:

• there's definitely more - the tutorial should also mention some general concepts around CQRS and provide advice beyond "here are 11 techniques, you fail if you on't use at least 9 of them"
• address the key point is that one size does not fit all and its important that the Handler and Service be well thought through
  • if you end up cut and pasting exactly the same one as boilerplate per aggregate its a bad sign
  • if you end up using more than 2-3 of the techniques above, that's also a bad sign

Deferred:

• Command calling out to external decision process (e.g. if a decision process needs to examine the folded state, then propose a relevant command and/or have a longer chain of calls which you want to wrap an optimistic retry loop around)
  • There are Async overloads for prominent functions which result from specific cases in existing codebase. They will not be documented in Add Commands and Handlers Tutorial #96 as they don't constitute best practice worth highlighting (@eiriktsarpalis argues for removal of such temptations from the house entirely; for now they remain)

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Ping @Xatter would love feedback on #96 - its on the long side; I'd be keen to hear if you feel it addresses the reasons that led to your +1

I'm going to close this for now, but please feel free to log a "there's no docs for aspect X" or "docs should have guidance for how to think about well known design tradeoff X when using Equinox" issues