Instructions for building on MacOS

[rvjansen]

it might be a good idea to add this to the instructions to save someone ten minutes on stack exchange

[stweil]

Building training on macOS Big Sur with prereqs from homebrew requires brew link icu4c --force

I never use that and suggest to use an autoconf build with this configuration instead:

configure --disable-shared 'CXXFLAGS=-g -O2 -Wall' PKG_CONFIG_PATH=$(brew --prefix)/opt/icu4c/lib/pkgconfig:$(brew --prefix)/opt/libarchive/lib/pkgconfig:$(brew --prefix)/Library/Homebrew/os/mac/pkgconfig/11