Update README.md

Author: analogrelay

Xamarin/README.md

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 ## Xamarin Clients
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+**NOTE:** This sample currently does not work because of a blocking issue in the System.Memory package. See https://github.com/aspnet/SignalR/issues/1886 for more information. This issue is being resolved in an upcoming patch release.
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 The server application is located in the `ChatSample` folder.
 
 Be sure to enter your own server url in the hub connection builder.