Link to DIT documentation in README (#148)

Author: adrhill

DifferentiationInterface/README.md

@@ -1,9 +1,13 @@
 # DifferentiationInterface
 
-[![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://gdalle.github.io/DifferentiationInterface.jl/DifferentiationInterface/dev/)
 [![Build Status](https://github.com/gdalle/DifferentiationInterface.jl/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/gdalle/DifferentiationInterface.jl/actions/workflows/CI.yml?query=branch%3Amain)
 [![Code Style: Blue](https://img.shields.io/badge/code%20style-blue-4495d1.svg)](https://github.com/invenia/BlueStyle)
 
+| Package                      | Documentation                              |
+|:-----------------------------|:------------------------------------------:|
+| DifferentiationInterface     | [![Dev][docs-dev-badge]][docs-dev-url-DI]  |
+| DifferentiationInterfaceTest | [![Dev][docs-dev-badge]][docs-dev-url-DIT] |
+
 An interface to various automatic differentiation (AD) backends in Julia.
 
 ## Goal
@@ -75,3 +79,8 @@ julia> value_and_gradient(f, backend, [1., 2., 3.])
 
 - [AbstractDifferentiation.jl](https://github.com/JuliaDiff/AbstractDifferentiation.jl) is the original inspiration for DifferentiationInterface.jl.
 - [AutoDiffOperators.jl](https://github.com/oschulz/AutoDiffOperators.jl) is an attempt to bridge ADTypes.jl with AbstractDifferentiation.jl.
+
+[docs-dev-badge]: https://img.shields.io/badge/docs-dev-blue.svg
+[docs-dev-url-DI]:  https://gdalle.github.io/DifferentiationInterface.jl/DifferentiationInterface/dev/
+[docs-dev-url-DIT]: https://gdalle.github.io/DifferentiationInterface.jl/DifferentiationInterfaceTest/dev/
+