chore: add contributing guidelines for bigframes.bigquery (#16972)

Add contributing guidelines for bigframes.bigquery APIs, detailing input
and output policies, naming conventions, and examples for SQL
operations.

Thank you for opening a Pull Request! Before submitting your PR, there
are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a
[bug/issue](https://github.com/googleapis/google-cloud-python/issues)
before writing your code! That way we can discuss the change, evaluate
designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: tswast

packages/bigframes/noxfile.py

@@ -605,11 +605,11 @@ def prerelease(session: nox.sessions.Session, tests_path, extra_pytest_options=(
         # Workaround https://github.com/googleapis/python-db-dtypes-pandas/issues/178
         "db-dtypes",
         # Ensure we catch breaking changes in the client libraries early.
-        "git+https://github.com/googleapis/python-bigquery.git#egg=google-cloud-bigquery",
+        "git+https://github.com/googleapis/google-cloud-python.git#egg=google-cloud-bigquery&subdirectory=packages/google-cloud-bigquery",
         "--upgrade",
         "-e",
         "git+https://github.com/googleapis/google-cloud-python.git#egg=google-cloud-bigquery-storage&subdirectory=packages/google-cloud-bigquery-storage",
-        "git+https://github.com/googleapis/python-bigquery-pandas.git#egg=pandas-gbq",
+        "git+https://github.com/googleapis/google-cloud-python.git#egg=pandas-gbq&subdirectory=packages/pandas-gbq",
     )
 
     # Print out prerelease package versions.