Update style guidelines in CONTRIBUTING.md

[emmanuel-ferdman]

Why is this the best possible solution? Were any other approaches considered?

Small PR - commit 1f71ade moved the CONTRIBUTING.md file to docs. But still, the CONTRIBUTING.md does not provide the style guidelines. The CODE-GUIDELINES.md does, so this PR updates the CONTRIBUTING.md to reference CODE-GUIDELINES.md.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Do we need any specific form for testing your changes? If so, please attach one.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Before submitting this PR, please make sure you have:

• added or modified tests for any new or changed behavior
• run ./gradlew connectedAndroidTest (or ./gradlew testLab) and confirmed all checks still pass
• added a comment above any new strings describing it for translators
• added any new strings with date formatting to DateFormatsTest
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.
• verified that any new UI elements use theme colors. UI Components Style guidelines

[emmanuel-ferdman]

@seadowg thank you for the review, fixed 😄

[seadowg]

Thanks so much @emmanuel-ferdman!