Clarify how additive changes are handled (#455)

This PR explicitly lists the additive changes allowed and adds
a requirement that such additive changes must be accompanied by
interoperability explanation when necessary.

This is a subset of #432
that contains other guarantees that we did not yet agree to.
I believe this particular subset is necessary regardless of
what we decide about #432.

README.md

@@ -66,6 +66,19 @@ The following changes are allowed:
 Note that none of the above allowed changes affects the binary wire representation or the
 JSON wire representation.
 
+The following additive changes are allowed:
+
+- Adding new fields to existing messages.
+- Adding new messages or enums.
+- Adding new choices to existing enums.
+- Adding new choices to existing oneof fields.
+- Adding new services.
+- Adding new methods to existing services.
+
+All the additive changes above must be accompanied by an explanation about how
+new and old senders and receivers that implement the version of the protocol
+before and after the change interoperate.
+
 No guarantees are provided whatsoever about the stability of the code that
 is generated from the .proto files by any particular code generator.