update osx build instructions

[zanuka]

There are issues getting core build installed and running based on the notes here: https://github.com/NAVCoin/navcoin-core/blob/master/doc/build-osx.md

• update to ensure a better dev workflow

[marcus290]

@matt-auckland can we close this?

[mxaddict]

@aguycalled is this document for OSX up to date?