README could be better examplining how to run things

[neojski]

It seems not obvious as how to compile the example at the tip. I managed to do it by:

sed 's/~rest:`Log//' -i **/*.ml
mv example/dune{.example,}
dune build example/hello_world.so
emacs -Q -L _build/default/example --batch --eval "(progn (require 'hello_world) (say-hello \"emacs\"))"

I suspect that an alternative would be to run v0.14 to compile against the async that's in opam so that I wouldn't need to drop rest. What's the recommended way of running those at home?