Rewrite documentation

[ianfixes]

At minimum, the following changes are needed:

• Convert document to markdown format
• Split README.md into more relevant parts
  • GETTINGSTARTED.md - how to configure your environment
  • CONTRIBUTING.md - how to develop, test, submit changes, etc
  • TROUBLESHOOTING.md - possible gotchas
• Pull in any valid information from https://www.emaculation.com/doku.php/compiling_sheepshaver_basilisk