chore: updating CONTRIBUTING.md

[suztomo]

Updating CONTRIBUTING.md as per go/java-repos-contrib-guidelines

b/323273913

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 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 # ☕️

If you write sample code, please follow the samples format.

[generated-files-bot[bot]]

Warning: This pull request is touching the following templated files:

• CONTRIBUTING.md