WIP: 2.0 Upgrade Guide

- [ ] Legacy Riak Search to Yokozuna. Upgrade process detailed [here](https://github.com/basho/yokozuna/blob/develop/riak_test/yz_rs_migration_test.erl#L6) and in #1087. `advanced.config` settings must be documented #1088 
- [ ] Explain `allow_mult` behavior
- [ ] Caveats about bucket types
- [ ] Cuttlefish + old system support
- [ ] When there's no turning back
- [ ] Detail expected disk usage for Yokozuna vs legacy search. Findings documented in basho/yokozuna#343