Making go-local-coauthors-plus a bit more ready for public consumption

- Added a README.md file
- Tweaked some conditionals so `coauthors_posts_links` doesn't fall over in a non GO environment.
- Going forward autoload is set to 'no' on the cache option.
This probably brings up the point that we _should_ split off the good functionality in this plugin (read: everything except this function) into its own plugin: `go-coauthors-plus` and then keep `go-local-coauthors-plus` for our custom stuff that isn't useful to share.

Yes, I know @misterbisson wants to do our own coauthor plugin, but that's a bigger thing.