Lookup map: update readme

pkg/lookup_map/README.md

 # Lookup maps
 
 This package contains the definition of `LookupMap`: a simple, but very
 restricted map. The map can only hold constant keys and the only way to use the
 map is to retrieve values with a key you already have.  Expect for lookup, any
 other operation in `Map` (like forEach, keys, values, length, etc) is not
 available.
 
 Constant `LookupMap`s are understood by dart2js and can be tree-shaken
 internally: if a key is not used elsewhere in the program, its entry can be
 deleted from the map during compilation without changing the program's behavior.
 Currently dart2js supports tree-shaking keys that are Type literals, and any
 const expression that can only be created with a const constructor. This means
 that primitives, Strings, and constant objects that override the `==` operator
 cannot be tree-shaken.
 
+**Note**: this feature is currently experimental in dart2js, we recommend trying
+other alternatives before relying on this feature.
 
 ## Examples
 
 `LookupMap` is unlikely going to be useful for individual developers writing
 code by hand. It is mainly intended as a helper utility for frameworks that need
 to autogenerate data and associate it with a type in the program. For example,
 this can be used by a dependency injection system to record how to create
 instances of a given type. A dependency injection framework can store in a
 `LookupMap` all the information it needs for every injectable type in every
 library and package.  When compiling a specific application, dart2js can
 tree-shake the data of types that are not used by the application. Similarly,
 this can also be used by serialization/deserialization packages that can store
 in a `LookupMap` the deserialization logic for a given type.