Document required node version in CONTRIBUTING.md

[rjmk]

The doctest for

> S.encaseEither(S.I, JSON.parse, '[')
Left(new SyntaxError('Unexpected end of input'))

will break as JSON.parse's error message changed at some point between node 4 and node 6 to "Unexpected end of JSON input"

We should document which node versions can be used by people developing the library

[davidchambers]

How do you feel about this now that #250 has been merged, @rjmk?

[rjmk]

Much better solution!