Add check for Hello World in README

[matthewfeickert]

I just realized that I don't think that we actually have a check anywhere for the "Hello World" example that is in the README.

>>> import pyhf
>>> import pyhf.simplemodels
>>> p = pyhf.simplemodels.hepdata_like(signal_data=[12.,11.], bkg_data=[50.,52.], bkg_uncerts=[3.,7.])
>>> *_, CLs_obs,CLs_exp = pyhf.runOnePoint(1.0, [51, 48] + p.config.auxdata, p)
>>> print('obs: {} exp: {}'.format(CLs_obs, CLs_exp[2]))
obs: [0.05290116] exp: [0.06445521]

If something was going to break the README example it would break elsewhere as well, but we wouldn't also realize that we need to update the README example. For example, the above should now be pyhf.utils.runOnePoint.

Unless there is an alternative suggestion I'll open a PR that will add a notebook to both the CI and to the docs.

[kratsg]

Doctesting should catch it if we add the readme in.

[matthewfeickert]

Ah, okay I'll add that in the PR. I didn't know that it could also get the README, but it is just another file (so duh yeah it can).