Update README.md (#7)

diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -1,5 +1,8 @@
 [![Build Status](https://travis-ci.org/dart-lang/characters.svg?branch=master)](https://travis-ci.org/dart-lang/characters)
 
+**NOTE**: This package is considered experimental, and breaking API changes are
+to be expected.
+
 `Characters` are strings viewed as sequences of *user-perceived character*s,
 also know as [Unicode (extended) grapheme clusters](https://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries).