Add documentation on how to use contract-helpers

[eternauta1337]

If one wants to use, for example, the test helpers, there is no indication on the readme that one should use the npm package "@aragon/contract-helpers-test". You have to go into the code, open the package.json in question, and figure out the package you need. Would be nice if there was a usage section in the readme.

EDIT: Actually, the package states @aragon/contract-helpers-test, but the package was published as @aragon/contract-test-helpers, which is even more confusing.