docs: add contributing guide to `README` (#33490)
**Description:** Added a beginner-friendly tip to the README to help
first-time contributors find a starting point. This is a documentation
improvement aimed at lowering the barrier for newcomers to participate
in open source.
**Issue:** No related issue
**Dependencies:** None
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## Note to maintainers
I'm new to open source and this is my first PR! If there's anything that
needs improvement, please guide me and I'll be happy to learn and make
changes. Thank you for your patience! 😊
## What does this PR do?
- Added a noticeable beginner tip box after the badges section in README
- Provided specific guidance (Good First Issues link)
- Encourages newcomers to start with documentation fixes
## Why is this change needed?
- Makes it easier for new contributors to get started
- Provides clear direction and reduces confusion
- Creates a more welcoming open source community environment
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Co-authored-by: Mason Daugherty <mason@langchain.dev>
