contributing: Add CONTRIBUTING.md with release instructions

CONTRIBUTING.md

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+# Contributing
+
+Contributions of all kind and with all levels of experience are very welcome. Before working on big changes or features please open an issue first to discuss scope and implementation details.
+
+## Release
+
+To create a new release, follow these steps:
+
+- Update version number in Cargo.toml for library and CLI
+- Update Cargo.lock
+- Commit the changes
+- Build binaries with `cargo build --release` for `--target x86_64-pc-windows-gnu` and `--target x86_64-unknown-linux-gnu`
+- Tag the commit: `git tag -as a.b.c`
+- Push the tag: `git push origin a.b.c`
+- Create a github release for the tag and upload the built binaries
+- Push a new crate version to crates.io with `cargo publish -p magic-wormhole`