Include some helpful tips while building using `stack` in README

As of now someone who tries to include morley as a dependency to their stack project will quickly face difficulty due to morley dependencies not being on stackage or hackage. The suggested solution for this issue is to copy extra-deps from the repo's stack.yaml to their own stack configuration.

If we don't want to face the wrath of users who wants to give morley libraries like cleveland a try, we should include a warning about this in the README, and also suggest they try copying the extra-deps from morley to their own projects stack.yaml configuration.