Update contributing with BuildNet9 info

CONTRIBUTING.md

@@ -2,8 +2,30 @@
 
 1. Clone repo.
 2. Install local tools with `dotnet tool restore`
+
+## Testing
+
+Testing against different .NET runtimes is a bit of a chore because we want to let the runtime load the current runtime's Microsoft.Build assemblies. Unfortunately, we can't have the `global.json` set to `net9.0` because the `Microsoft.Build` assemblies are compatible with the .NET 9 runtime, and will try to load them, even in a net8.0 TFM context. (Chet to fill in longer description as to why.)
+
+### Against LTS (net8.0)
+1. Run tests with `dotnet run --project .\build\ -- -t Test`
+    1. This should chose the `Test:net9.0` target and run against that runtime
+
+
+### Against STS (net9.0)
+1. Change global.json to use net9.0
+    ```json
+        "sdk": {
+            "version": "9.0.100",
+            "rollForward": "latestMinor",
+            "allowPrerelease": true
+        }
+    ```
+2. Set environment variable `BuildNet9` to `true`
+    1. Bash: `export BuildNet9=true`
+    2. PowerShell: `$env:BuildNet9 = "true"`
 3. Run tests with `dotnet run --project .\build\ -- -t Test`
-4. Create packages with `dotnet run --project .\build\ -- -t Pack`
+    1. This should chose the `Test:net9.0` target and run against that runtime
 
 ## Release