Update README.md

Author: ChrisRackauckas

README.md

@@ -81,3 +81,12 @@ development.
 - AFArrays
 - MultiScaleArrays
 - LabelledArrays
+
+## Breaking Release Notes
+
+2.0: Changed the default of `ismutable(array::AbstractArray) = true`. We previously defaulted to 
+`Base.@pure ismutable(array::AbstractArray) = typeof(array).mutable`, but there are a lot of cases
+where this tends to not work out in a way one would expect. For example, if you put a normal array
+into an immutable struct that adds more information to it, this is considered immutable, even if
+all of the `setindex!` methods work (by forwarding to the mutable array). Thus it seems safer to just
+always assume mutability is standard for an array, and allow arrays to opt-out.