recommend using rule definition tools in docs

docs/src/writing_good_rules.md

@@ -67,6 +67,40 @@ julia> rrule(foo, 2)
 While this is more verbose, it ensures that if an error is thrown during the `pullback` the [`gensym`](https://docs.julialang.org/en/v1/base/base/#Base.gensym) name of the local function will include the name you gave it.
 This makes it a lot simpler to debug from the stacktrace.
 
+## Use rule definition tools
+
+Rule definition tools can help you write more `frule`s and the `rrule`s with less lines of code.
+
+### [`@non_differentiable`](@ref)
+
+For non-differentiable functions the [`@non_differentiable`](@ref) macro can be used.
+For example, instead of manually defining the `frule` and the `rrule` for string concatenation `*(String..)`, the macro call
+```
+@non_differentiable *(String...)
+```
+defines the following `frule` and `rrule` automatically
+```
+function ChainRulesCore.frule(var"##_#1600", ::Core.Typeof(*), String::Any...; kwargs...)
+    return (*(String...; kwargs...), DoesNotExist())
+end
+function ChainRulesCore.rrule(::Core.Typeof(*), String::Any...; kwargs...)
+    return (*(String...; kwargs...), function var"*_pullback"(_)
+        (Zero(), ntuple((_->DoesNotExist()), 0 + length(String))...)
+    end)
+end
+```
+Note that the types of arguments are propagated to the `frule` and `rrule` definitions.
+This is needed in case the function differentiable for some but not for other types of arguments.
+For example `*(1, 2, 3)` is differentiable, and is not defined with the macro call above.
+
+### [`@scalar_rule`](@ref)
+
+For functions involving only scalars, i.e. subtypes of `Number` (no `struct`s, `String`s...), both the `frule` and the `rrule` can be defined using a single [`@scalar_rule`](@ref) macro call. 
+
+Note that the function does not have to be $\mathbb{R} \rightarrow \mathbb{R}$.
+In fact, any number of scalar arguments is supported, as is returning a tuple of scalars.
+
+See docstrings for the comprehensive usage instructions.
 ## Write tests
 
 In [ChainRulesTestUtils.jl](https://github.com/JuliaDiff/ChainRulesTestUtils.jl)