README could be better examplining how to run things

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

Start with the README and reproduce the commands in the issue, including the changes to example/dune.example and the dune build example/hello_world.so command. Check whether the documented setup should use the current tip or v0.14 with the opam async package. Done means the README explains the recommended way to compile and run example/hello_world.so from a local checkout.

Written by the indexing model from the issue text.