Update Building for Core CLR.md

docs/compilers/Building for Core CLR.md

@@ -18,7 +18,11 @@ powershell script
 [src/Tools/CopyCoreClrRuntime/CopyCoreClrRuntime.ps1](../../src/Tools/CopyCoreClrRuntime/CopyCoreClrRuntime.ps1) 
 with the path to the output directory as the parameter.
 
-If you then wish to build the csc project with the CoreCLR compiler, simply set
-the CscToolPath environment variable to the core-clr path on your machine and
-the CscToolExe variable to `csc.exe`. Finally, rebuild the csc -- this should
-build csc.exe with the CoreCLR-compatible csc.exe.
+### Dogfooding the compiler
+
+If you then wish to dogfood the CoreCLR copmilers while developing Roslyn then set 
+the BootstrapBuildPath environment variable (or at the MSBuild command line) to 
+point to the core-clr directory.
+
+> msbuild /p:BootstrapBuildPath=<path-to-core-clr-dir>
+