Update README

README.mdwn

@@ -7,6 +7,8 @@ It's heavily inspired by Sisyphus (and uses some of its code)
 
 __Please contribute test cases.__
 
+__Semantic test cases that are executable (and terminate) should be placed in the `exec` folder.__
+
 *There's also some other additional code in this repo, that might provide fuzzing capabilities. Please consider contributing
 such code as it probably helps the other teams (and could later be integrated into a bigger test suite).*