Add note and link to issue

Author: devmotion

src/ChainRules.jl

@@ -56,6 +56,8 @@ function __init__()
         include("rulesets/packages/NaNMath.jl")
     end
 
+    # Note: drop SpecialFunctions dependency in next breaking release
+    # https://github.com/JuliaDiff/ChainRules.jl/issues/319
     @require SpecialFunctions="276daf66-3868-5448-9aa4-cd146d93841b" begin
         if !isdefined(SpecialFunctions, :ChainRulesCore)
             include("rulesets/packages/SpecialFunctions.jl")