Add details about dependencies

Author: nickrobinson251

docs/src/getting_started.md

@@ -1,8 +1,11 @@
 # Getting Started
 
 [ChainRulesCore.jl](https://github.com/JuliaDiff/ChainRulesCore.jl) is a light-weight dependency for defining sensitivities for functions in your packages, without you needing to depend on ChainRules itself.
+It only depends on [Cassette.jl](https://github.com/jrevels/Cassette.jl) which is also light-weight and has no dependencies.
 
 [ChainRules.jl](https://github.com/JuliaDiff/ChainRules.jl) provides the full functionality, including sensitivities for Base Julia and standard libraries.
+Sensitivities for some other packages, currently SpecialFunctions.jl and NanMath.jl, will also be loaded if those packages are in your environment.
+In general, we recommend adding custom sensitivities to your own packages with ChainRulesCore.
 
 ## Defining Custom Sensitivities