SSI MTR documentation

[Amy-Xu]

Just created a branch for all current versions of documentation: https://github.com/open-source-economics/Benefits/tree/docbranch/SSI/MTR

We need to replace the current version with a more simplified version that describes 1) what method was used, and 2) what was the results. If other contributors ever want to contribute to this portion of Basic Income project, documentation work can be added easily.

Thanks @parkerrogers

[parkerrogers]

Done

[Amy-Xu]

Thanks!