Building on macOS Mojave

Just wanted to share how I got the HTML to build on macOS Mojave 10.4.5.

Dependencies:

* ruby 2.3.8 (I used [asdf-ruby](https://github.com/asdf-vm/asdf-ruby) to install it then [set the asdf ruby version locally](https://asdf-vm.com/#/core-manage-versions?id=set-current-version))
* python 2.7.16 (I used [asdf-python](https://github.com/danhper/asdf-python) to install it then [set the asdf python version locally](https://asdf-vm.com/#/core-manage-versions?id=set-current-version))
* pygments.rb 0.6.3 (`gem install pygments.rb -v 0.6.3`)
* asciidoctor 1.5.4 (`gem install asciidoctor -v 1.5.4`)

Then use `make html`. To get the HTML to look right I had to move `dist/index.html` to the root of the git checkout and then open it (so `./assets/*` and `./images/*` could be found by the browser).

(**NOTE**: It might pay for the Makefile to copy `./assets` and `./images` into `./dist` for the `html` target.)

Read the Makefile and the existing build documentation, then run `make html` using the dependency versions listed in the issue. Check whether the generated HTML can locate `assets` and `images` from `dist`; done means the macOS build instructions or build behavior clearly support opening the generated HTML.

Written by the indexing model from the issue text.