Add High Level Design Document

Change-Id: I2d1d1cee342ae3f74bb6dab1cc73febd74aa0818
Sample change

Change-Id: I9747a9d39f77cebc079b56c47ce36f798407e107
Add images and source yed files for diagrams

Also moved overview.md to design sub-folder

Change-Id: I317112e995ab40649da8204941d5ba172e6f21d8
Add System Diagram for Multiple Buckets/Indexes

Also created sub-documents for deployment, mutation, query
and terminology and linked from the main (overview.md)

Better exported svgs for mutation workflows

Change-Id: I9bf0aab557e9edd023594f5fd082e48cb266b57d