Adding some descriptions on how to get asciidoctor-diagram to act like a...

... commandline replacement for asciidoctor
As I went through your instructions I found it confusing as to how I could use this to work from the commandline.  Looking over your examples and combining with the existing asciidoctor wrapper, I put together these instructions.

In my use, I actually put ~/bin in my path, and create a symlink asciidoctor to point to the dasciidoctor wrapper.  This way asciidoctor is still the command that I call.
Updated the README and added the examples/dasciidoctor script.