Update README with some basic instructions.

Notes:
`sx-tab` should not need to be required.  The necessary functions should be auto-loaded.
Could this be changed to the following?
 =ready= column or the [[=ready= label][https://github.com/vermiculus/sx.el/issues?q=is%3Aopen+is%3Aissue+label%3Aready+-label%3A%22in+progress%22]]