Add Diffractor backend to README (#98)

Author: gdalle

README.md

@@ -33,6 +33,9 @@ The following backends are temporarily made available by `AbstractDifferentiatio
 - `AD.ZygoteBackend()` for [Zygote.jl](https://github.com/FluxML/Zygote.jl), which is a special case of `AD.ReverseRuleConfigBackend`
 
 In the long term, these backend objects (and many more) will be defined within their respective packages to enforce the `AbstractDifferentiation` interface.
+This is already the case for some of them:
+
+- `Diffractor.DiffractorForwardBackend()` for [Diffractor.jl](https://github.com/JuliaDiff/Diffractor.jl) in forward mode
 
 Here's an example: