Add an implementation warning to Shrink (addresses #6)

shrink.go

@@ -5,8 +5,14 @@ import (
 	"reflect"
 )
 
-// Shrink is a stream of shrinked down values
-// Once the result of a shrink is false, it is considered to be exhausted
+// Shrink is a stream of shrinked down values.
+// Once the result of a shrink is false, it is considered to be exhausted.
+// Important notes for implementors:
+//   * Ensure that the returned stream is finite, even though shrinking will
+//     eventually be aborted, infinite streams may result in very slow running
+//     test.
+//   * Ensure that modifications to the returned value will not affect the
+//     internal state of your Shrink. If in doubt return by value not by reference
 type Shrink func() (interface{}, bool)
 
 // Filter creates a shrink filtered by a condition