Create a document describing the approach to upgrading

We need to document how we will upgrade edx-platform and make recommendations on how to upgrade from Django 1.8 to 1.11.

This document should talk about the incremental approach and providing support for multiple versions of Django at the same time.

A/C:

• Document created

• shared with the team

This document should include:

• how to deal with upgrading requirements

• testing and merging strategy for incremental feature changes