add "required examples" specification

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