updated README to include macOS build instructions (also fixed some wrong formattings)

(thanks for this! been looking for an updated version of mtn for ages)

just tested; builds (and works ofc) like a charm on macOS!

If you're ok with it, I'm also going create a brew "package", fix some other stuff I've noticed (like for example the test/run_mtn.sh script) and look into building qmtn

also I'm not too sure about your git release/branch/tag model... Looks like git flow, so I've put my changes in devel branch.

Let me know if I should change something