DOCSP-42568 Cluster B Size for ABC Migration (#393)

* DOCSP-42568 Cluster Size note for ABC Migration

* Title adjustment

Author: kennethdyer

source/reference/beta-program/abc-migration.txt

@@ -58,6 +58,13 @@ For better performance, ensure that the first migration (A->B) reaches
 see if a migration reached ``change event application``, check the ``info`` 
 field in the :ref:`c2c-api-progress` response document.
 
+Cluster B Size
+~~~~~~~~~~~~~~
+
+Writes from cluster A and reads from cluster C can result in heavy loads for
+cluster B. Ensure that cluster B is sufficiently sized and can handle simultaneous
+read and write loads.
+
 Migration Name 
 ~~~~~~~~~~~~~~