Add FAQ about CockroachDB and CAP

[jseldess]

We need an FAQ to clarify how CockroachDB is high availability and strongly consistent without violating the CAP theorem.

See for further discussion: https://docs.google.com/document/d/1wsQbbYpVEHa2zr1qjCNMxDI_1a9vjBIWIw6G5M0put0/edit

cc: @jess-edwards, @spencerkimball, @bdarnell, @petermattis