Improve the "contributing documentation" readme

Update the "contributing documentation" readme to include the correct
gradle goal and provide a troubleshooting section.
Hard-wrap CONTRIBUTING-DOCUMENTATION at 90 chars
Merge pull request #434 from snicoll/ascidoc-liveupdate

* ascidoc-liveupdate:
  Hard-wrap CONTRIBUTING-DOCUMENTATION at 90 chars
  Improve the "contributing documentation" readme