Delete README.md

Moving real readme here
Update and rename readme.txt to README.md
Update README.md
Update README.md
Merge pull request #1 from OASIS-XDI-Technical-Committee/josephboyle

Josephboyle
Merge pull request #2 from windley/master

Moved readme.txt to README.md
Mostly skeleton version including ABNF
Catchup with Drummond yesterday changes
Merge branch 'master' into josephboyle
Fix refs and syntax, explain abnf
Reorganize and add more descriptive text