Mention const field of mutable struct

doc/src/manual/types.md

@@ -459,6 +459,27 @@ To recap, two essential properties define immutability in Julia:
  functions as pointers to heap-allocated values except in cases where the compiler
  is sure that there's no way to tell that this is not what is happening.
 
+In cases where one or more fields of an otherwise mutable struct is known to be immutable,
+one can declare these fields as such using `const` as shown below. This enables some,
+but not all of the optimizations of immutable structs, and can be used to enforce invariants
+on the particular fields marked as `const`.
+
+```jldoctest baztype
+julia> mutable struct Baz
+ a::Int
+ const b::Float64
+ end
+
+julia> baz = Baz(1, 1.5);
+
+julia> baz.a = 2
+2
+
+julia> baz.b = 2.0
+ERROR: setfield!: const field .b of type Baz cannot be changed
+[...]
+```
+
 ## [Declared Types](@id man-declared-types)
 
 The three kinds of types (abstract, primitive, composite) discussed in the previous