More readme tweaks
More readme tweaks

Change weight of subheadings

Move build section above os x and environment variables


Former-commit-id: 82362615e821f3bdeb1994f1236c293d6b8af7c1
darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide