Merge pull request #905 from AvaloniaUI/900-add-note-about-license-key-must-not-be-blank

Add warning about blank license key

Author: luke-whos-here

accelerate/installation.md

@@ -30,4 +30,8 @@ Include your Avalonia UI license key in the executable project file (`.csproj`):
 </ItemGroup>
 ```
 
-> For multi-project solutions, you can store your license key in an [environment variable](https://learn.microsoft.com/en-us/visualstudio/msbuild/how-to-use-environment-variables-in-a-build) or a [shared props file](https://learn.microsoft.com/en-us/visualstudio/msbuild/customize-by-directory?view=vs-2022#directorybuildprops-example) to avoid duplication.
+For multi-project solutions, you can store your license key in an [environment variable](https://learn.microsoft.com/en-us/visualstudio/msbuild/how-to-use-environment-variables-in-a-build) or a [shared props file](https://learn.microsoft.com/en-us/visualstudio/msbuild/customize-by-directory?view=vs-2022#directorybuildprops-example) to avoid duplication.
+
+:::warning
+Do not leave the license key value blank. If left blank, you may be unable to build or open your project.
+:::