clarify what deriving packages there are

[agarwal]

Minor suggestions to avoid users going on a wild goose chase:

• [ ] Right now this project is marked as being a fork of jaked/deriving. As resolved in #5, this is now the master fork, so that is just confusing. I would disassociate this project from jaked/deriving. I believe you have to contact GitHub support to make this request, but they are very responsive so that should be easy.
• [ ] The README links to http://code.google.com/p/deriving/, but again this seems completely out of date. Please delete the link.
• [ ] This project used to be distributed as opam package deriving-ocsigen, but is now called deriving. So thus the opam package description for deriving-ocsigen should say "DEPRECATED: please use deriving".