[docs](trx-rs): adopt kernel-style AI attribution trailers

Replace the guidance to credit LLM usage via 'Co-authored-by:' with the
Linux kernel convention. Co-Authored-By and Co-Developed-By are reserved
for human authors (who must also sign off); AI/LLM assistance is
disclosed with an Assisted-By: trailer instead.

Apply the new policy to this commit as a worked example.

Assisted-By: Claude Code (claude-opus-4)
Claude-Session: https://claude.ai/code/session_01NFpGtGTWUEYXLwZeZs2RAV
Signed-off-by: Stan Grams <sjg@haxx.space>

CONTRIBUTING.md

@@ -18,7 +18,7 @@ When contributing to the project, please follow these guidelines:
 - Use a maximum of 80 characters per line.
 - Use a blank line between the commit message and the body.
 - Sign your commits with `git commit -s`.
-- Explicitly mark LLM usage in commit messages with 'Co-authored-by:'.
+- Disclose AI/LLM assistance with an `Assisted-By:` trailer (see below).
 
 Use the format below for commit titles:
 [<type>](<crate>): <description>
@@ -39,3 +39,23 @@ Allowed types:
 - chore: build or maintenance changes
 
 Write isolated commits for each crate.
+
+## Attribution trailers
+
+This project follows the Linux kernel convention for crediting work.
+
+The `Co-Authored-By:` and `Co-Developed-By:` trailers name **people** who
+authored the change. They are reserved for humans, and every person named
+this way must also add their own `Signed-off-by:` line. Never use these
+trailers for tools, assistants, or bots.
+
+When a commit was produced with help from an AI assistant or LLM,
+disclose it with an `Assisted-By:` trailer naming the tool (and model,
+where relevant). The human committer remains the author of record and
+takes responsibility for the change through `Signed-off-by:`.
+
+Example:
+
+    Assisted-By: Claude Code (claude-opus-4)
+    Co-Authored-By: Jane Developer <jane@example.com>
+    Signed-off-by: Your Name <you@example.com>