docs: Add info about tap creation

[andy9a9]

Tap directories for furmulae/casks have to be created manually. Add additional step into docu for creating them.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?

Closes #15640

[EricFromCanada]

Nice work! Just adjusting some wording to clarify what the word "tap" means in this context.

[EricFromCanada]

It's through contributions like yours that we've been able to run this project. Thanks, @andy9a9!