docs: add license badge to README
Add a license badge next to the existing Open Bounties badge to make the project's license more visible to users and contributors.
Changes:
- Added license badge using shields.io that links to the project's LICENSE file
Testing:
- No testing was required as this is a documentation-only change
- Verified the badge markdown syntax is correct and follows the existing badge format
Link to Devin run: https://app.devin.ai/sessions/ac29b93bbf6e476a9e82e3ce35a540a6
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