Add basic CLAUDE.md file

We add a basic `CLAUDE.md` file pointing it to some workspace specifics
(in particular running `cargo fmt` on 1.75.0 MSRV, and using
`./ci/ci-tests.sh` to run tests).

Signed-off-by: Elias Rohrer <dev@tnull.de>

Author: tnull

CLAUDE.md

@@ -0,0 +1,17 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+See [CONTRIBUTING.md](CONTRIBUTING.md) for build commands, testing, code style, and development workflow.
+
+## Workspace Structure
+
+See [README.md](README.md) for the workspace layout and [ARCH.md](ARCH.md) for some additional remark regarding important parts of LDK's architecture.
+
+## Development Rules
+
+- Always ensure tests pass before committing. To this end, you should run the test suite via `./ci/ci-tests.sh`.
+- Run `cargo +1.75.0 fmt --all` after every code change
+- Never add new dependencies unless explicitly requested
+- Please always disclose the use of any AI tools in commit messages and PR descriptions using a `Co-Authored-By:` line.
+- When adding new `.rs` files, please ensure to always add the licensing header as found, e.g., in `lightning/src/lib.rs` and other files.