Add simple reform example to docs about policyengine-us HOT 1 OPEN

@MaxGhenis, when I click on the "two examples" [of a reform] in issue #56, I get a 404 error saying the page does not exist.

But I very much agree with the idea of the issue:

Let's start with a very simple one, like changing the payroll tax rate.

Perhaps you can add a pytest that does that and then simply point users/developers to that reform test in the README.md file. Currently there is absolutely no documentation, and when a user/developer looks for guidance from OpenFisca-Core the only reform examples are in French.

from policyengine-us.