Compiling on MacOS Catalina + Homebrew: update README.md

[snickell]

This updates README.md with detailed and well tested instructions for compiling python-mapnik on MacOS Catalina systems using the very common homebrew packages.

This is a surprisingly difficult task as of Sep 2020, with a few time consuming gotchas. Three of us have spent a couple days each getting this working, seemed worth documenting at the README level for others, without the detailed docs python-mapnik is not casually installable on OSX for most people who are not really determined.

[snickell]

It seems like PRs aren't getting merged much on this repo, for anyone coming here who's been struggling to install on MacOS, I created build-python-mapnik-on-osx.sh: https://gist.github.com/snickell/ee8f3d96f11f457c30ef5416a12d625e

Note: This will compile mapnik 4 and install it, so it uninstalls the older mapnik 3.0 from homebrew