change file tree (#1859)

Co-authored-by: Chayenne <zhaochenyang@g.ucla.edu>

docs/references/contributor_guide.md

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+# Contributor Guide
+
+## Format Your Code
+Use these commands to format your code and pass CI linting tests.
+
+```
+pip3 install pre-commit
+cd sglang
+pre-commit install
+pre-commit run --all-files
+```
+
+## Add Unit Tests
+Add unit tests under [sglang/test](https://github.com/sgl-project/sglang/tree/main/test). You can learn how to add and run tests from the README.md in that folder.