docs: Add dependency and version change guidelines

CONTRIBUTING.md

@@ -314,3 +314,35 @@ python -m pip install --upgrade awkward
 python -m pip uninstall --yes awkward awkward-cpp
 python -m pip install --upgrade --extra-index-url https://pypi.anaconda.org/scientific-python-nightly-wheels/simple awkward
 ```
+
+### Guidelines for Dependency and Version Changes
+
+To ensure stability and reproducibility across the Awkward ecosystem:
+
+1. Collaborative Review Required • Pull requests that affect core dependencies (e.g., Numba, Pandas, PyArrow, NumPy) or Python version support must be discussed with maintainers before merging. • This avoids unilateral decisions that could break critical workflows.
+
+2. Compatibility Transparency• If a dependency is not yet available for a new Python release, note this clearly in the PR description and documentation.
+• Example: “Supports Python 3.14, but Numba-dependent features are disabled until upstream support is released.”
+
+3. Testing Expectations• CI should run against all supported Python versions.
+• Tests that depend on unsupported features (e.g., Numba on Python 3.14) must be marked with xfail so contributors see the limitation.
+
+4. Version Pinning• Pin or constrain versions of critical dependencies to prevent silent breakages from upstream changes.
+• Floating versions may be allowed for secondary utilities, but core scientific packages should be locked.
+
+5. Communication• Use GitHub Discussions or Issues to propose changes before opening a PR that alters compatibility.
+• This ensures consensus and avoids surprises for downstream users.
+
+
+
+---
+
+#### Example PR Checklist
+
+• Have I discussed this change with maintainers?
+
+• Did I document limitations (e.g., missing Numba support)?
+
+• Are CI tests updated to reflect compatibility status?
+
+• Did I pin or constrain critical dependencies?