Add docs for using AbstractDifferentiation

Author: oxinabox

docs/src/index.md

@@ -3,3 +3,32 @@
 Next-generation AD
 
 [PDF containing the terminology](terminology.pdf)
+
+## Getting Started
+
+⚠️This certainly has bugs and issues. Please open issues on [Diffractor.jl](https://github.com/JuliaDiff/Diffractor.jl/), or [AbstractDifferentiation.jl](https://github.com/JuliaDiff/AbstractDifferentiation.jl/issues) as appropriate.⚠️
+
+Diffractor's public API is via [AbstractDifferentiation.jl](https://github.com/JuliaDiff/AbstractDifferentiation.jl/).
+Please see the AbstractDifferentiation.jl docs for detailed usage.
+
+```@jldoctest
+julia> using Diffractor: DiffractorForwardBackend
+
+julia> using AbstractDifferentiation: derivative
+
+julia> derivative(DiffractorForwardBackend(), +, 1.5, 10.0)
+(1.0, 1.0)
+
+julia> derivative(DiffractorForwardBackend(), *, 1.5, 10.0)
+(10.0, 1.5)
+
+julia> jacobian(DiffractorForwardBackend(), prod, [1.5, 2.5, 10.0]) |> only
+1×3 Matrix{Float64}:
+ 25.0  15.0  3.75
+
+julia> jacobian(DiffractorForwardBackend(), identity, [1.5, 2.5, 10.0]) |> only
+3×3 Matrix{Float64}:
+ 1.0  0.0  0.0
+ 0.0  1.0  0.0
+ 0.0  0.0  1.0
+```