Update readme with out-of-tree build instructions
Update readme with out-of-tree build instructions


Former-commit-id: 56beee2b943de1cd7a10e87f0be946be218fa6c0
Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide