Add summary to package readme

Author: kzu

src/WebSocketeer/readme.md

@@ -1,3 +1,6 @@
+A thin, intuitive, idiomatic and high-performance API for 
+Azure Web PubSub [protobuf subprotocol](https://docs.microsoft.com/en-us/azure/azure-web-pubsub/reference-protobuf-webpubsub-subprotocol).
+
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