README: Describe osek build process more explicitly

README.md

@@ -23,6 +23,8 @@ The other thing that's needed to build mockups in dosek is
 In the dosek source, use the following to get a working build
 environment that knows and uses the mockup generator:
 
+	mkdir build
+	cd build
     CMAKE_INCLUDE_PATH=<location-of-mockup-prelude> ../new_build_env.py -a posix --generate-mockup t
 	
 where `<location-of-mockup-prelude>` is the directory containing