Add note about mongosync perf issue when Balancer is enabled (#243) (#246)

* add admonition regarding getBalancerState

* reword warning and re-include

* fix link to getBalancerState()

* fix writing issues

* move note location

* move to new section

* Update source/includes/sharding-perf.rst



---------

Co-authored-by: Jeff Allen <jeffrey.allen@10gen.com>

Author: MongoCaleb

source/includes/sharding-perf.rst

@@ -0,0 +1,12 @@
+
+If the destination is a sharded cluster with the balancer enabled, write 
+performance of the destination cluster may degrade during ``mongosync``'s 
+Collection Copy phase. To warn of this potential performance impact,   
+``mongosync`` runs the :method:`sh.getBalancerState()` command during 
+initialization. If ``getBalancerState()`` returns ``true``, ``mongosync`` 
+logs a warning. 
+
+If write performance during migration is impacted, consider disabling the 
+balancer on the destination cluster. You stop the balancer with the 
+:dbcommand:`balancerStop` command, and restart it with the 
+:dbcommand:`balancerStart` command.

source/includes/sharding-warning.rst

@@ -169,6 +169,13 @@ Use ``curl`` or another HTTP client to issue the :ref:`start
 The ``start`` command options must be the same for all of the ``mongosync``
 instances.
 
+.. _c2c-sharded-performance:
+
+Performance
+-----------
+
+.. include:: /includes/sharding-perf
+
 .. _c2c-sharded-progress:
 
 Check Progress
@@ -300,4 +307,3 @@ instance that is running on ``mongosync01Host`` and using ``port
    Reverse synchronization is only possible if ``reversible`` and
    ``enableUserWriteBlocking`` are both set to ``true`` when the
    :ref:`start API <c2c-api-start>` initiates ``mongosync``.
-