Update README

Author: josesimoes

README.md

@@ -6,6 +6,8 @@
 
 ### Welcome to the .NET **nanoFramework** System.Net.WebSockets Library repository
 
+This API mirrors (as close as possible) the official .NET [System.Net.WebSockets](https://docs.microsoft.com/en-us/dotnet/api/system.net.websockets.clientwebsocket). Exceptions are mainly derived from the lack of `async` and generics support in .NET **nanoFramework**.
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 | Component | Build Status | NuGet Package |