Update README.md (dotnet#12787)

Add a note about VB.NET

README.md

@@ -46,6 +46,8 @@ What would not make the bar:
 * Functionality, which would "stretch" a Windows Desktop App to be a mobile, Multi-Media or IoT app.
 * Domain-specific custom controls, which are already provided by the vast variety of third party control vendors
 
+**A note about Visual Basic**: Visual Basic .NET developers make up about 20% of WinForms developers. We welcome changes that are specific to VB if they address a bug in a customer-facing scenario. Issues and PRs should describe the customer-facing scenario and, if possible, include images showing the problem before and after the proposed changes. Due to limited bandwidth, we cannot prioritize VB-specific changes that are solely for correctness or code cleanliness. However, VB remains important to us, and we aim to fix any critical issues that arise.
+
 ## Please note
 
 :warning: This repository contains only implementations for Windows Forms for [.NET platform](https://github.com/dotnet/core).<br />