Make a README

[nskinkel]

Should probably contain, at a minimum:

• info about official tweetnacl (from the paper, probably)
• links to official tweetnacl
• explanation of what this is and how it's different from NaCL and libsodium
• some notes on design decisions
• usage caveats
• credit to original authors
• list of alternate implementations
• info about unsafe usage, thread safety, etc.

[dwtj]

A stub README.md added as of commit 7f2f0eb5d920bf125cd1c7cf60647fc7b7b8eff2.

All of these sound like good material for a proper README.md. Two simple additions:

• Dependency information (i.e. mention how cargo takes care of things and that a copy of tweetnacl source is included).
• Install information (i.e. basic cargo instructions).