docs(contributing); fix node compat and more
docs(contributing); fix node compat and more

Fix current Node version support policy, now that only Node 4 and
higher are supported.

While we're in CONTRIBUTING, also fix up the commit message guidelines,
clarifying that the subject should not be captialized, and modify
the list of acceptiable scopes. `markdown` should never have been used
as a scope; `lint` should have been used instead; and add two new
scopes.

daxelrod/jowl