Update README: expand macOS and Linux issue #34

[bamboo512]

在 README 添加了 #34 #37，因此 macOS、GNU/Linux 用户不必查看 issue 即可知道如何解决无法使用 login 登录的问题：

A type initializer threw an exception. To determine which type, inspect the InnerException's StackTrace property.

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Merged #34 #37 into README so that user no longer need to look up issues when faced with logging in bug:

A type initializer threw an exception. To determine which type, inspect the InnerException's StackTrace property.

[hez2010]

System.Drawing.Common has dropped support for non-Windows platforms since .NET 6, so BBDown currently doesn't support login from non-Windows platforms even if you have mono-libgdiplus installed. We're waiting for updates from QRCoder.

[hez2010]

Workaround: https://docs.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/6.0/system-drawing-common-windows-only#recommended-action