Add documentation on how to test

[policyengine]

Add documentation on how to test

[nikhilwoodruff]

We should add this to the documentation I agree, but to run the tests, the command is currently openfisca-us test openfisca_us/tests. I just filed #36 which updates the Makefile to run that command - at which point the command will be make test (as we'll need to run other tests, some Python, some OpenFisca).

[MaxGhenis]

Thanks, that worked.

[martinholmer]

@MaxGhenis, about a year ago you raised issue #35 asking how to run the pytests, and after @nikhilwoodruff responded with adding a test target to the Makefile, you responded:

Thanks, that [i.e., make test] worked.

Is there more to do on issue #35? If you think there should be more documentation, how about adding a help target to the Makefile (that explains what all the Makefile targets do) and telling users/developers in the README.md file to enter make help to get more information?