make pdf on OSX howto

[Michael Trouw]

For any OSX user with outdated system ruby gem + X11:

brew install ruby
which gem # /usr/local/bin/gem
gem -v # 2.6.2. in my case
(sudo) gem install asciidoctor
which asciidoctor # returned binary path
make git # no errors
make pdf # warnings, and X11 install popup

good god, X11 again:

brew install Caskroom/cask/xquartz # completes without error
make pdf # completes without errors, though pops up invisible java thingy
open -a Preview dist/clojurescript-unraveled.pdf

Awesome!

[Deleted user]

Cool, we could add these instructions to the README for OS X users.

[Michael Trouw]

yw !

[Andrey Antukh]

Added reference on the readme.

[Cora Sutton]

Updated build instructions for Mojave: #153