[CT-1593] [Bug] Update core/dbt/README.md to match current situation

[yoiki]

Is this a new bug in dbt-core?

• I believe this is a new bug in dbt-core
• I have searched the existing issues, and I could not find an existing issue for this bug

Current Behavior

There are several omissions in core/dbt/README.md . constants.py, selected_resources.py, etc. are not listed.

Expected Behavior

List some file name under ## The following are individual files in this directory.

• constants.py
• selected_resources.py

Add some dir name under ## The subdirectories will be documented in a README in the subdirectory

• contracts
• parser
• tests

It may be a good idea to sort this list in abc order to improve readability.

Steps To Reproduce

Please check core/dbt/README.md

Relevant log output

No response

Environment

- OS:
- Python:
- dbt:

Which database adapter are you using with dbt?

No response

Additional Context

No response

[dbeatty10]

Thanks for noticing and reaching out @yoiki !

I created #6371 to incorporate your suggestions.