add note about 0.15b0 release
## Unreleased
+## Version 0.15b0
+
+Released 2020-11-02
+
+- Rewrite gRPC server interceptor
+ ([#1171](https://github.com/open-telemetry/opentelemetry-python/pull/1171))
+
## Version 0.13b0
Released 2020-09-17
