[docs](trx-rs): add CONTRIBUTING.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

CONTRIBUTING.md

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+## Workflow
+
+The trx-rs project is organized into three main components:
+1. **trx-core**: core library providing the basic functionalities,
+2. **trx-server**: server component that hooks up the transceiver,
+3. **trx-client**: client component that connects with the trx-server and exposes selected frontends.
+
+When contributing to the project, please follow these guidelines:
+- Fork the repository and create a new branch for your changes.
+- Make sure your code follows the project's coding style and conventions.
+- Write clear and concise commit messages.
+- Submit a pull request with a detailed description of your changes.
+- Ensure that your changes are tested and pass all existing tests.
+
+## Commit Guidelines
+- Use imperative mood in commit messages.
+- Keep commit messages short and descriptive.
+- 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:'.
+
+Use the format below for commit titles:
+[<type>](<crate>): <description>
+e.g.
+- [feat](trx-core): add new feature xyz
+- [fix](trx-frontend): fix http frontend xyz issue
+- [docs](trx-rs): update README
+
+Use `(trx-rs)` for repo-wide changes that are not specific to any crate.
+
+Allowed types:
+- feat: new feature
+- fix: bug fix
+- docs: documentation changes
+- style: code style changes
+- refactor: code refactoring
+- test: test changes
+- chore: build or maintenance changes
+
+Write isolated commits for each crate.