Add note about updating an existing installation (dotnet#45518)

Author: captainsafia

docs/BuildFromSource.md

@@ -39,6 +39,7 @@ Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser
 1. In order to install Visual Studio on your machine, you can use the official installer script in the repo.
 
 > :warning: Even if you have the appropriate Visual Studio installed on your machine, we recommend running this installation script to make sure that the correct Visual Studio components are installed.
+> To modify an existing Visual Studio installation, follow [the instructions for installing from a configuration file](https://learn.microsoft.com/en-us/visualstudio/install/import-export-installation-configurations?view=vs-2022#import-a-configuration) and use the `.vsconfig` file located in the root of the repository.
 
 ```powershell
 ./eng/scripts/InstallVisualStudio.ps1