Announce Moped EOL

[alexbevi]

Moped has not been the official Ruby driver for MongoDB for almost a decade now. Since all development is targeted at https://github.com/mongodb/mongo-ruby-driver and Mongoid stopped using moped long ago it's long overdue that this project be archived.

I'm proposing archiving this repository to make it read-only and updating the README to point developers to the active Ruby driver for MongoDB instead.

I don't anticipate any objections to this decision, but wanted to open it up for discussion just in case.