Add info to CONTRIBUTING about how to setup the repo

[ChaelCodes]

Add a really short and basic description of how to setup the development environment. Even though it's very brief, I think it's helpful for newer OSS contributors and over-thinkers to have a dev setup guide in the contributing guide.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• [-] Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [-] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• [-] If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[ChaelCodes]

Happy to update this with any feedback or suggestions about how this section could be changed!