[linting] Cleaning up linting documentation

[john-bodley]

Per @timifasubaa's suggestion I updated the linting logic in CONTRIBUTING.md. Simply running flake8 or (npm run lint for that matter) doesn't work off the bat as one needs to i) install all the flake8 extensions, and ii) cd to superset/assets respectively.

For the flake8 case given that the configuration logic resides in the tox.ini file it seemed to make more sense to direct people to invoke tox locally for linting both Python and Javascript code. The later required an additional tox test environment which is not invoked by Travis since the javascript environment is a superset of eslint.

[xrmx]

Wondering if flake8 deps should go into dev-reqs.txt

[john-bodley]

@xrmx I see you point. Couldn't one also argue that from a testing perspective instead of having flake8, pylint, etc. defined in dev-reqs.txt, we should merely have tox, and then tox.ini would handle all the testing dependencies?

[xrmx]

I suspect not many people run tox on their machine, but just rely on CI. Either way it's good for me.

[mistercrunch]

I also think of tox as something CI-related. I'd prefer having the flake8 config in .flake8 and telling people to run the flake8 command. It makes it more clear. With tox it's unclear what you're actually running and you have to follow the redirection to know what's up.