Add manual testing instructions

[seanh]

Add instructions to HACKING.md for how to test your local development version of this tox plugin in tox.

I don't think it makes sense to try to make this any easier (for example by trying to wrap the whole thing in a convenient make command): we only maintain a few tox plugins and they're all really simple ones that hopefully won't require much ongoing development. Maybe we can try just living with manual instructions for this in HACKING.md for now.

Once we merge this I'll add the same to the tox.ini files of tox-faster and tox-envfile. We could add this to the pypackage cookiecutter but it only applies to tox plugins so we'd have to add a tox_plugin boolean setting to cookiecutter.json (similar to other boolean settings like console_script). Might not be worth adding this to the cookiecutter since it only applies to tox plugins?

[marcospri]

Is make sure missing a sure: python (the python bit)

I had to call bin/make_python manually

[seanh]

Yes, that looks like a bug in the cookiecutter. Opened an issue: hypothesis/cookiecutters#48

[marcospri]

Yeah, this is very useful specially in repos were we don't work on very often.

There's a missing dependency on make sure definition but that probably needs to be fixed via the cookiecutter + PRs to re-apply it.