Create README.md (copy of README.TXT in markdown)

(To make the Github page more readable)
A few edits for accuracy and clarity

* OSX -> Mac OS X
* Mac OS X 7 is required to run Java 7
* "Script Editor" is now called "AppleScript Editor"
Additional formatting/editting

* Add Getting Java section
* Add links to required software
* Add instructions for building jgnash-help
* Add location of final .zip file
* Formatting improvements