add warning to README.md (#1810)

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## Description
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Add the warning to `README.md` until #1659 is resolved.

## Motivation and Context
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<!--- If it fixes an open issue, please link to the issue here. -->
The WinUI 3 Gallery can not be built. Add the warning to `README.md`
until #1659 is resolved.

## How Has This Been Tested?
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to -->
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https://github.com/manabu-nakamura/WinUI-Gallery/tree/warning#3-open-winuigallerysln-with-visual-studio-and-build

## Screenshots (if appropriate):

![s](https://github.com/user-attachments/assets/426c5bcb-ba0f-4817-8a0c-17ac8d6abc9a)

## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Other

Author: manabu-nakamura

README.md

@@ -63,6 +63,12 @@ git clone https://github.com/microsoft/WinUI-Gallery.git
 
 Ensure that the `WinUIGallery` project is set as the startup project in Visual Studio.
 
+> [!WARNING]
+> Try deleting [`nuget.config`](nuget.config) and building again if you get the following build error:
+> > Assets file 'C:\Users\\...\source\repos\WinUI-Gallery\WinUIGallery\obj\WinUIGallery\project.assets.json' not found. Run a NuGet package restore to generate this file.
+>
+> See [issue #1659: Broken repo build](https://github.com/microsoft/WinUI-Gallery/issues/1659).
+
 ## ➡️ Further information
 
 To learn more about Windows app development, go to the [Windows Dev Center](https://developer.microsoft.com/windows).