Add mac install instructions and dust link

It seems like mac users are running into install problems.  Added a pointer to the common issue and workaround.

Also added a section to link to dust.
This could be simplified (the path is repeated, for some reason), and without exporting:
```
PKG_CONFIG_PATH='/usr/local/Cellar/libffi/3.0.13/lib/pkgconfig' ./make-with-jit
```
I've gone this route each time, and it's worked well.