add warning message to isimmutable documentation

Author: ssikdar1

base/deprecated.jl

@@ -194,7 +194,8 @@ end
 # BEGIN 1.5 deprecations
 """
     isimmutable(v) -> Bool
-
+!!! warning
+    Consider using `!ismutable(v)` instead, as `isimmutable(v)` will be replaced by `!ismutable(v)` in a future release.
 Return `true` iff value `v` is immutable.  See [Mutable Composite Types](@ref)
 for a discussion of immutability. Note that this function works on values, so if you give it
 a type, it will tell you that a value of `DataType` is mutable.