add "required examples" specification

[mattsb42-aws]

Add a specification doc that details the required examples, as laid out here[1].

Things we need to define include:

• The examples readme structure.
  • This is the structure and content laid out here[1].
• The list of all example scenarios.
  • This is the complete list of examples defined here[1].
• Requirements for any given example.
  • Every example MUST be independent.
  • Every example MUST NOT require additional or prior knowledge to understand.
  • Every example MUST explain step by step what it is doing.
  • Those explanations MUST differentiate between things that we are recommending that customers do vs things that we are demonstrating to customers.

[1] https://github.com/aws/aws-encryption-sdk-python/tree/master/examples

[robin-aws]

Another less obvious example to include: demonstrate that it is possible and encouraged to create multiple keyrings/master key providers in a single application. Our existing examples demonstrate encrypting a single message with multiple CMKs, but I haven't found any that demonstrate creating a separate keyring for each customer according to the context. We could label this a "multi-tenancy" example.