Feature/unity asset store

[longda]

Asana Ticket

Description

1. Adds readme files to various folders to maintain project structure when generating .unitypackage for Unity Asset Store
2. Adds a menu option to generate a full project .unitypackage for Unity Asset Store distribution of MDK

Motivation and Context

1. Provide an alternative option for users to install this repo as MDK from the Unity Asset Store

How has this been tested?

1. Tested locally with a fresh, clean, empty project. Can build artifacts such as Playground Files.
2. Can manually upload the above files to Playground.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.