Update readme - asciibinder.net no longer exists

[samccann]

While trying to follow the readme for building docs etc, I came across two problems:

1. The mention of asciibinder points to a dead url. Google search comes up with a redhat resource that says it's no longer maintained. Possibly just replace with asciidoctor?
2. The section on Building the guide tells the reader to go to the main directory, but there is no master.adoc there. Instead, the reader needs to cd to the guide they want to build within the titles/ directory.