Write docs on how to write rules to run different computations at different times

With #30
this might be a great example

function rrule(::typeof(/), A::AbstractVecOrMat{<:Real}, B::AbstractVecOrMat{<:Real})
    # Compute C (return value differently) to just calling `A/B`
    # so we have some parts precomputed 
    # that we need to use for computing the pullback

    Aᵀ, dA_pb = rrule(adjoint, A)
    Bᵀ, dB_pb = rrule(adjoint, B)
    Cᵀ, dS_pb = rrule(\, Bᵀ, Aᵀ)
    C, dC_pb = rrule(adjoint, Cᵀ)
    
    function slash_pullback(Ȳ)
         # These two run no matter which partial you want
         # but they don't run if you only want the result (C)

        _, dC = dC_pb(Ȳ)
        _, dAᵀ, dBᵀ = dS_pb(dC)

        # Each of these only run if you want that particular partial
        ∂A = @thunk last(dA_pb(dAᵀ))
        ∂B = @thunk last(dA_pb(dBᵀ))

        (NO_FIELDS, ∂A, ∂B)
    end
    return C, slash_pullback
end

Start by reading linked issue #30 and the rrule example in this issue, focusing on which computations run for the result versus requested partials. Done means documentation explains how to write rules with this timing behavior and makes the example understandable; no documentation file or test is named in the issue.