Ready: 2.0 Upgrade Guide

- [x] Advise using new client libraries
- [x] 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 
- [x] Explain `allow_mult` behavior
- [x] Caveats about bucket types
- [x] Cuttlefish + old system support
- [x] When there's no turning back
- [x] Detail expected disk usage for Yokozuna vs legacy search. Findings documented in basho/yokozuna#343
I'm not sure these points need numbered :). 

"...if you have created and activated a bucket type."