[#907] Update `README.md` to include `extra-deps`

Description

Problem: 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.

Solution: Add a recommendation to morley's README.md for the user to include the extra-deps if they have difficulty building morley.

Related issue(s)

Resolves #907 (closed)

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• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
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