diff --git a/src/encoding/encoding.go b/src/encoding/encoding.go
index cc5a53699661d..50acf3c23a101 100644
--- a/src/encoding/encoding.go
+++ b/src/encoding/encoding.go
@@ -9,6 +9,12 @@
 // interface once can make a type useful in multiple encodings.
 // Standard types that implement these interfaces include time.Time and net.IP.
 // The interfaces come in pairs that produce and consume encoded data.
+//
+// Adding encoding/decoding methods to existing types may constitute a breaking change,
+// as they can be used for serialization in communicating with programs
+// written with different library versions.
+// The policy for packages maintained by the Go project is to only allow
+// the addition of marshaling functions if no existing, reasonable marshaling exists.
 package encoding
 
 // BinaryMarshaler is the interface implemented by an object that can