Document example version constraint in README.
diff --git a/README.md b/README.md
index 81f6467..8ec4483 100644
--- a/README.md
+++ b/README.md
@@ -14,7 +14,7 @@
 to your `pubspec.yaml` file:
 
     dependencies:
-      fixnum: any
+      fixnum: '>=0.9.1 <1.0.0'
 
 Then run `pub install`.