When installing via .pkg, you can place a file in /etc/paths.d that adds to the path. For example, when you install in /usr/local/rethink/, you just add a file (e.g. /etc/paths.d/rethink) with one line: /usr/local/rethink/bin. Thats it, very easy.
Congrats on the new release! You are definitely doing a great job making developers love RethinkDB, feels like you already generated loads of interest.
I'll try to comment here as well when this is done!