chore: update contributing guide (#1337)

## 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:
- [ ] 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/langchain-google-alloydb-pg-python/issues/new/choose)
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)
- [ ] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

CONTRIBUTING.md

@@ -51,10 +51,10 @@ are inactive for more than 30 days may be closed.
 
 ## Adding a New Database Source or Tool
 
-We recommend creating an
+Please create an
 [issue](https://github.com/googleapis/genai-toolbox/issues) before
-implementation to ensure we can accept the contribution and no duplicated work.
-If you have any questions, reach out on our
+implementation to ensure we can accept the contribution and no duplicated work. This issue
+should include an overview of the API design. If you have any questions, reach out on our
 [Discord](https://discord.gg/Dmm69peqjh) to chat directly with the team.  
 
 > [!NOTE]
@@ -227,4 +227,4 @@ 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`).