update testing docs

[satra]

as noted in #1905 CONTRIBUTING.md does not directly provide users with how to test nipype easily on their system. it perhaps should point to this link http://nipype.readthedocs.io/en/latest/devel/testing_nipype.html, which should be updated. especially this section:
http://nipype.readthedocs.io/en/latest/devel/testing_nipype.html#testing-nipype-using-docker

[effigies]

As I don't test using Docker, am not familiar with the intended usage of the Docker hub images, and would need to become familiar with both, I wouldn't be able to write this quickly. Is this something someone can put together quickly?

Also, should we go ahead, start a release branch and test on conda-forge? If the only thing changing is docs, there's little chance of test-breaking changes.

[oesteban]

I can have a look on this next week. But impossible for me right now

[djarecka]

@oesteban - I can start updating and ask you for review once you have time.

[effigies]

Sounds good. Docs update on master, so doesn't seem necessary for release.

[effigies]

Related to #2313 (comment) Q1, what is the intended usage of the docker hub images? Are these intended for testing? Or are users expected to build the main Dockerfile on top of nipype/nipype:base?

[satra]

@effigies - i use nipype/nipype:py36 for my local development/testing. so for those using docker, those docker hub images give people the exact environment that's needed for nipype testing.

[oesteban]

Same here. Would be great to take the specific command lines from the CircleCI scripts into this piece of docs.

[effigies]

We should update CONTRIBUTING.md to recommend pip install rather than python setup.py.

From #2471 (comment).

[djarecka]

@effigies so you want to have just pip install -e . instead of python setup.py? or should we have pip install -e .[all] or pip install -e .[specs]?