ValueSerializer: Add a warning about making wire format changes near branch.

src/value-serializer.cc

Index: src/value-serializer.cc
diff --git a/src/value-serializer.cc b/src/value-serializer.cc
index 44cb8dc1887720374db4fb5dd92413c21cfab67a..04978d8f64f0f74456cda31b31df23a038d75e67 100644
--- a/src/value-serializer.cc
+++ b/src/value-serializer.cc
@@ -29,6 +29,14 @@ namespace internal {
 // Version 12: regexp and string objects share normal string encoding
 // Version 13: host objects have an explicit tag (rather than handling all
 //             unknown tags)
+//
+// WARNING: Increasing this value is a change which cannot safely be rolled
+// back without breaking compatibility with data stored on disk. It is
+// strongly recommended that you do not make such changes near a release
+// milestone branch point.
+//
+// Recent changes are routinely reverted in preparation for branch, and this
+// has been the cause of at least one bug in the past.
 static const uint32_t kLatestVersion = 13;
 
 static const int kPretenureThreshold = 100 * KB;