[FAB-1759] [FAB-1703] Doc couchDB

Added the required CouchDB doc for V1 Fabric.
What is, why, when needed etc.

Change-Id: Ia827f3a284ee1ecaab9a8457c78860fca6592b57
Signed-off-by: luomin <luomin_tokyotech@hotmail.com>

docs/source/build_network.rst

@@ -895,6 +895,24 @@ The output should display the two marbles owned by ``jerry``:
 
        Query Result: [{"Key":"marble2", "Record":{"color":"red","docType":"marble","name":"marble2","owner":"jerry","size":50}},{"Key":"marble3", "Record":{"color":"blue","docType":"marble","name":"marble3","owner":"jerry","size":70}}]
 
+
+Why CouchDB
+-------------
+CouchDB is a kind of NoSQL solution. It is a document oriented database where document fields are stored as key-value mpas. Fields can be either a simple key/value pair, list, or map.
+In addition to keyed/composite-key/key-range queries which are supported by LevelDB, CouchDB also supports full data rich queries capability, such as non-key queries against the whole blockchain data,
+since its data content is stored in JSON format and fully queryable. Therefore, CouchDB can meet chaincode, auditing, reporting requirements for many use cases that not supported by LevelDB.
+
+CouchDB can also enhance the security for compliance and data protection in the blockchain. As it is able to implement field-level security through the filtering and masking of individual attributes within a transaction, and only authorizing the read-only permission if needed.
+
+In addition, CouchDB falls into the AP-type (Availability and Partition Tolerance) of the CAP theorem. It uses a master-master replication model with ``Eventual Consistency``.
+More information can be found on the
+`Eventual Consistency page of the CouchDB documentation <http://docs.couchdb.org/en/latest/intro/consistency.html>`__.
+However, under each fabric peer, there is no database replicas, writes to database are guaranteed consistent and durable (not ``Eventual Consistency``).
+
+CouchDB is the first external pluggable state database for Fabric, and there could and should be other external database options. For example, IBM enables the relational database for its blockchain.
+And the CP-type (Consistency and Partition Tolerance) databases may also in need, so as to enable data consistency without application level guarantee.
+
+
 A Note on Data Persistence
 --------------------------