Update README with some basic instructions.

Notes:
`sx-tab' should not need to be required.  The necessary functions should
be auto-loaded.
Refill to 80 characters
More complete README
Add CONTRIBUTING.org
Link to `ready' issues
Merge pull request #132 from vermiculus/update-readme

Update README with some basic instructions.
Merge branch 'master' into answering
Merge branch 'answering' into fix-@highligthing