Provide Microsoft.AspNetCore.SignalR.Client.Core package README (#57817)

Author: MackinnonBuck

src/SignalR/clients/csharp/Client.Core/src/PACKAGE.md

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+## About
+
+`Microsoft.AspNetCore.SignalR.Client.Core` provides core functionality for the .NET client for ASP.NET Core SignalR.
+
+> [!NOTE]
+> This package contains only the connection-agnostic components of the .NET SignalR client, and does not provide a default connection implementation. In most scenarios, the [`Microsoft.AspNetCore.SignalR.Client`](https://www.nuget.org/packages/Microsoft.AspNetCore.SignalR.Client) package should be used because it provides an HTTP connection implementation.
+
+## Key Features
+
+SignalR provides the following capabilities:
+* Automatic connection management
+* Sending messages to all connected clients simultaneously
+* Sending messages to specific clients or groups of clients
+* Scaling to handle increasing traffic
+
+## How to Use
+
+To use `Microsoft.AspNetCore.SignalR.Client.Core`, follow these steps:
+
+### Installation
+
+```shell
+dotnet add package Microsoft.AspNetCore.SignalR.Client.Core
+```
+
+### Configuration
+
+The .NET SignalR client requires a connection implementation. To use an HTTP connection implementation, install the  [`Microsoft.AspNetCore.SignalR.Client`](https://www.nuget.org/packages/Microsoft.AspNetCore.SignalR.Client) package.
+
+## Main Types
+
+The main types provided by `Microsoft.AspNetCore.SignalR.Client.Core` include:
+* `HubConnectionBuilder`: Provides an abstraction to construct new SignalR hub connections
+* `HubConnection`: Defines methods for managing a hub connection, including:
+  * Starting and stopping the connection
+  * Sending and receiving messages
+  * Handling disconnects and attempting reconnects
+* `HubConnectionOptions`: Provides options for configuring a `HubConnection`
+
+## Additional Documentation
+
+For additional documentation and examples, refer to the [official documentation](https://learn.microsoft.com/aspnet/core/signalr/dotnet-client) on the .NET client for ASP.NET Core SignalR.
+
+## Feedback & Contributing
+
+`Microsoft.AspNetCore.SignalR.Client.Core` is released as open-source under the [MIT license](https://licenses.nuget.org/MIT). Bug reports and contributions are welcome at [the GitHub repository](https://github.com/dotnet/aspnetcore).