Update CONTRIBUTING.md to include license information (#7512)

Added a section about licensing contributions to the project.

# Description of Changes

<!--
Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
-->

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

CONTRIBUTING.md

@@ -2,6 +2,13 @@
 
 Thank you for your interest in contributing to Stirling-PDF! There are many ways to contribute other than writing code. For example, reporting bugs, creating suggestions, and adding or modifying translations.
 
+## License
+
+By contributing to this project, you agree that your contributions will be licensed under the project [license](LICENSE), which follows an open-core model. 
+The codebase is a mix of MIT and source-available code, so your contribution is licensed according to the directory it is committed to.
+
+PRs are welcome in any directory by any user, just be aware of which license applies to the code you change.
+
 ## Issue Guidelines
 
 Issues can be used to report bugs, request features, or ask questions. If you have a question, you could also ask us in our [Discord](https://discord.gg/FJUSXUSYec).
@@ -63,7 +70,3 @@ For technical guides, setup instructions, and development resources:
 For configuration and usage guides, see:
 - [Database Guide](DATABASE.md) - Database setup and configuration
 - [OCR Guide](HowToUseOCR.md) - OCR setup and configuration
-
-## License
-
-By contributing to this project, you agree that your contributions will be licensed under the [MIT License](LICENSE).