update and reformat README.md

• updated references to files which have since been moved
• added reference to licensing
• added brief summary in case a non-technical user happens upon repo first
• miscellaneous Markdown-isms to make the doc more attractive
• remove unused symlink README -> README.md

I double-checked the makefiles and whatnot to ensure that the README symlink is
not being referenced. It is not. Rather, doc/README and
doc/README_windows.txt are copied for distribution.