Create README.md in Github markdown format, content editing and additions/improvements.

It's great to see jGNash on GitHub.  I thought a conversion to Markdown (.md) format of the README would greatly improve the appearance of the GitHub project page.

I also made some corrections and additions to the content.

I'm not sure if you want to delete README.TXT, because it is used on SourceForge, etc.  Let me know what you want to do...