docs: add info on Automated Code Reviews (#2031)

## Description

> Should include a concise description of the changes (bug or feature),
it's
> impact, along with a summary of the solution

## PR Checklist

> 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:

- [x] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [ ] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)
- [ ] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

CONTRIBUTING.md

@@ -48,6 +48,17 @@ squashed when merged.
 author for more than 10 days, maintainers may mark that PR as Draft. PRs that
 are inactive for more than 30 days may be closed.
 
+### Automated Code Reviews
+
+This repository uses **Gemini Code Assist** to provide automated code reviews on Pull Requests. While this does not replace human review, it provides immediate feedback on code quality and potential issues.
+
+You can manually trigger the bot by commenting on your Pull Request:
+
+*   `/gemini`: Manually invokes Gemini Code Assist in comments
+*   `/gemini review`: Posts a code review of the changes in the pull request
+*   `/gemini summary`: Posts a summary of the changes in the pull request.
+*   `/gemini help`: Overview of the available commands
+
 ## Adding a New Database Source or Tool
 
 Please create an
@@ -233,4 +244,4 @@ resources.
 * **PR Description:** PR description should **always** be included. It should
   include a concise description of the changes, it's impact, along with a
   summary of the solution. If the PR is related to a specific issue, the issue
-  number should be mentioned in the PR description (e.g. `Fixes #1`).
+  number should be mentioned in the PR description (e.g. `Fixes #1`).