Can't Compile Under macOS

[cdeyoung]

The README isn't very clear on all the dependencies. It mentions needing cmake, but not Ninja and Golang. The only way I could get etls to compile on macOS was to install cmake and ninja via Homebrew, and Golang via the asdf package manager.

I hope this helps anyone else who is having issues compiling etls on macOS.

P.S. I had to install the same three dependencies on Ubuntu to get etls to compile while using edeliver (ninja-build on Ubuntu 16.04, not the ninja package). It would be good to mention these dependencies in the README and how to install them for each OS.