A document describing invariants in the system.

rules of index partitioning is updated.
Invariants for index topology.

Change-Id: Ife53bdad49628a6ea148a0801d7780db6971848f
Reviewed-on: http://review.couchbase.org/34850
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>