Add instructions on using the check-upstream skill

Author: timsaucer

README.md

@@ -312,6 +312,33 @@ There are scripts in `ci/scripts` for running Rust and Python linters.
 ./ci/scripts/rust_toml_fmt.sh
 ```
 
+## Checking Upstream DataFusion Coverage
+
+This project includes a [Claude Code](https://claude.com/claude-code) skill for auditing which
+features from the upstream Apache DataFusion Rust library are not yet exposed in these Python
+bindings. This is useful when adding missing functions, auditing API coverage, or ensuring parity
+with upstream.
+
+To use it, run the `/check-upstream` slash command inside Claude Code with an optional area argument:
+
+```
+/check-upstream scalar functions
+/check-upstream aggregate functions
+/check-upstream window functions
+/check-upstream dataframe
+/check-upstream session context
+/check-upstream all
+```
+
+If no argument is provided, it defaults to checking all areas. The skill will fetch the upstream
+DataFusion documentation, compare it against the functions and methods exposed in this project, and
+produce a coverage report listing what is currently exposed and what is missing.
+
+The skill definition lives in `.claude/skills/check-upstream/SKILL.md`. Note that the `/check-upstream`
+slash command is a [Claude Code](https://claude.com/claude-code) feature, but the underlying
+methodology described in the skill file can be followed manually or by another AI coding agent if
+directed to read and follow the instructions in that file.
+
 ## How to update dependencies
 
 To change test dependencies, change the `pyproject.toml` and run