Include a history of UMD and when/why to use it

[DanKaplanSES]

I have no prior knowledge of UMD and was trying to learn more about it. This repository comes up first in a Google search, but the information in the readme was difficult for me to understand because it seemed to be written for somebody who already knew when and why to use UMD.

I would like to propose changes to the readme that explains UMD to those with no prior knowledge of it.

For additional context, see my stackoverflow question. The PR is a near copy/paste of the answer @bergus gave.