Clarify current stability guarantees

This PR captures the current understanding of stability guarnatees.
I want to make sure it is explicitly visible until we make a decision
on open-telemetry#400

The most likely outcome of open-telemetry#400
is going to be stronger than current definition of "Stable.

README.md

@@ -39,13 +39,33 @@ All messages                         | Alpha    |
 (See [maturity-matrix.yaml](https://github.com/open-telemetry/community/blob/47813530864b9fe5a5146f466a58bd2bb94edc72/maturity-matrix.yaml#L57)
 for definition of maturity levels).
 
-Note that maturity guarantees apply only to wire-level compatibility for the binary
-Protobuf serialization. Neither message, field, nor enum names of Protobuf messages
-are visible on the wire and are not considered part of the guarantees. We are free
-to make a change to the names.
+## Stability Definition
 
-In the future when OTLP/JSON is declared stable, field names will also become part of
-the maturity guarantees, since field names are visible on the wire for JSON encoding.
+Components marked `Stable` provide the following guarantees:
+
+- Field types will not change.
+- Field numbers will not change.
+- Numbers assigned to enum choices will not change.
+- Service names and service package names will not change.
+- Service operation names, parameter and return types will not change.
+
+The following changes are allowed:
+
+- Message names may change.
+- Field names may change.
+- Enum names may change.
+- Enum choice names may change.
+- The location of messages and enums, i.e. whether they are declared at the top
+  lexical scope or nested inside another message may change.
+- Package names may change.
+- Directory structure, location and the name of the files may change.
+
+No guarantees are provided whatsoever about the stability of the code that
+is generated from the .proto files by any particular code generator.
+
+In the future when OTLP/JSON is declared stable, several of the changes that
+are currently allowed will become disallowed since they are visible on the wire
+for JSON encoding.
 
 ## Experiments