[Mono] Add incremental build instructions when ... (dotnet#64396)

... you need to need to run library tests or run HelloWorld sample with your change to mono.

docs/workflow/building/mono/README.md

@@ -33,6 +33,16 @@ build.cmd mono
 ```
 When the build completes, product binaries will be dropped in the `artifacts\bin\mono\<OS>.<arch>.<flavor>` folder.
 
+If you need to run library tests or run HelloWorld sample with your change to mono, you want to build mono with this command instead:
+
+```bash
+./build.sh mono+libs.pretest
+```
+or on Windows,
+```cmd
+build.cmd mono+libs.pretest
+```
+
 If you want to skip restoring nuget packages, when only making change to mono, you want to use this command:
 ```bash
 ./build.sh mono --build