Add information about strong consistency directories to backup docs

[andrewjstone]

The data/ensembles directory should be included in the list of backup directories for people running 2.0 and using strong consistency.

[lucperkins]

@andrewjstone Do you know where SC data is stored on the OSes beyond OS X?

For example, on Ubuntu Bitcask data is stored in /var/lib/riak/bitcask. Would SC data by extension be stored in /var/lib/riak/ensemble?

[andrewjstone]

@lucperkins AFAIK, yes It should be at the same level as the bitcask directory, like your example. Except, it's ensembles not ensemble