Document MakeApiBaselines script (#4526)

docs/building.md

@@ -148,7 +148,8 @@ This sets up necessary environmental variables and opens the repository in VS Co
 * Windows Forms uses Visual Studio MSBuild but for certain features we require the latest MSBuild from .NET Core/.NET SDK. If you are on an official version of [Visual Studio][VS-download] (i.e. not a Preview version), then you may need to enable previews for .NET Core/.NET SDKs in VS.
   * you can do this in VS under Tools->Options->Environment->Preview Features->Use previews of the .Net Core SDK (Requires restart)
 * When restoring packages, if you get the following message: "The SSL connection could not be established, see inner exception. Unable to read data from the transport connection", try disabling IPv6 on your network adapter.
-
+* When the public API surface is updated, you need to update the API baselines manifest files for the impacted assemblies. [Here is an example](https://github.com/dotnet/extensions/blob/3e6ac0735cb62d4c8b63e131d956397a87ea26f7/src/Libraries/Microsoft.Extensions.AsyncState/Microsoft.Extensions.AsyncState.json) for the `Microsoft.Extensions.AsyncState` project. To update the manifest of all the local assemblies, invoke the script `./scripts/MakeApiBaselines.ps1`.
+```
 ---
 
 1. **"Solution"** means the collections of projects specified in `eng/build.proj` or an actual "sln" file at the root of the repository that represents the generated "filtered" solution (e.g., `SDK.sln`).

scripts/MakeApiBaselines.ps1

@@ -4,6 +4,12 @@
     Creates API baseline files representing the current API surface exposed by this repo.
 #>
 
+if ($PSVersionTable.PSVersion.Major -lt 6) {
+    Write-Host "PowerShell 6.0 or greater is required to run this script. See https://aka.ms/install-powershell."
+    Write-Host "Current version:" $PSVersionTable.PSVersion.ToString()
+    exit
+}
+
 $Project = $PSScriptRoot + "/../eng/Tools/ApiChief/ApiChief.csproj"
 $Command = $PSScriptRoot + "/../artifacts/bin/ApiChief/Debug/net8.0/ApiChief.exe"