collections.md: add ImmutableDict to list of dictionary types

Author: rfritz

doc/src/base/collections.md

@@ -186,6 +186,8 @@ thus may be garbage collected even when referenced in a hash table.
 Like `Dict` it uses `hash` for hashing and `isequal` for equality, unlike `Dict` it does
 not convert keys on insertion.
 
+ [`ImmutableDict`](@ref Base.ImmutableDict) is a linked list implementation.
+
 [`Dict`](@ref)s can be created by passing pair objects constructed with `=>` to a [`Dict`](@ref)
 constructor: `Dict("A"=>1, "B"=>2)`. This call will attempt to infer type information from the
 keys and values (i.e. this example creates a `Dict{String, Int64}`). To explicitly specify types