Add simple reform example to docs

[MaxGhenis]

Currently the two examples in https://ubicenter.github.io/openfisca-us/#reforms are:

1. From a large yaml file
2. Within a loop

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

[martinholmer]

@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.