doc(ci): add documentation for supply chain protection

supply-chain-protection.md

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+# Supply Chain Protection
+
+The CI for this repository uses the following tools to protect the supply chain:
+
+- [cargo-vet](https://github.com/mozilla/cargo-vet): vets dependencies based on existing audits with the aim of incrementally using fewer and fewer unaudited dependencies.
+- [cargo-deny](https://github.com/EmbarkStudios/cargo-deny): checks for unwanted licenses, crates, and other security issues.
+- [cargo-supply-chain](https://github.com/rust-secure-code/cargo-supply-chain): generates reports on dependencies and their authors.
+
+Below, we briefly explain how to configure these tools and how to make appropriate adjustments when dependencies change.
+
+## cargo-vet
+
+`cargo-vet` vets dependencies based on performed audits. In order to avoid redundant work, it encourages the use of already
+performed audits by trusted organizations or people. As of now, we trust audits performed by the
+[actix team](https://raw.githubusercontent.com/actix/supply-chain/main/audits.toml), the [bytecode-alliance](https://raw.githubusercontent.com/bytecodealliance/wasmtime/main/supply-chain/audits.toml)
+[embark-studios](https://raw.githubusercontent.com/EmbarkStudios/rust-ecosystem/main/audits.toml),
+[fermyon](https://raw.githubusercontent.com/fermyon/spin/main/supply-chain/audits.toml),
+[google](https://raw.githubusercontent.com/google/supply-chain/main/audits.toml),
+the [ISRG](https://raw.githubusercontent.com/divviup/libprio-rs/main/supply-chain/audits.toml),
+the [mozilla team](https://raw.githubusercontent.com/mozilla/cargo-vet/main/audits.toml),
+and the [ZCash foundaton](https://raw.githubusercontent.com/zcash/rust-ecosystem/main/supply-chain/audits.toml).
+Since, as of now, only a minority of crates have been audited, the tool aims at incrementally using fewer and fewer unaudited dependencies by initially exempting all dependencies
+from the need to be audited for the CI to pass. When more and more crates are audited, the tool prompts to reevaluate
+the list of exemptions and remove as many as possible.
+
+### Configuration
+
+The configuration files for cargo-vet are located in the `supply-chain` directory. The central configuration file is
+`config.toml`, where the lst of trusted organizations and the list of exemptions are defined.
+
+### Adding new dependencies
+
+Make sure to [install cargo vet](https://mozilla.github.io/cargo-vet/install.html) first.
+Then, when adding new dependencies, run `cargo vet suggest` before committing. If the dependency is not safe-to-deploy,
+add it to the exemptions in the file `supply-chain/config.toml`.
+
+For all further configration options, please refer to the [cargo-vet documentation](https://mozilla.github.io/cargo-vet/).
+
+## cargo-deny
+
+`cargo-deny` checks for unwanted licenses, crates, and other security issues. It is configured in the file `deny.toml`.
+
+### Licenses
+
+The only allowed licenses are the MIT license, the Apache-2.0 license, Apache-2.0 license WITH LLVM-exception,
+BSD-3-Clause license and the ISC license. All other licenses are banned. An exception is made for the
+`unicode-ident`-crate, for which we allow the Unicode-DFS-2016 license.
+
+### Security vulnerabilities
+
+The tool checks for security vulnerabilities in dependencies. If a vulnerability is found, the CI will fail. If you must,
+you can add exceptions in the `deny.toml` file, but this should only be a last resort.
+
+### Adding new dependencies
+
+Make sure to [install cargo deny](https://embarkstudios.github.io/cargo-deny/) first. Then, when adding new dependencies,
+run `cargo deny check` before committing. If there are issues wth the dependency reported by `cargo deny check`, you
+should try to resolve it appropriately. If this is not possible thoroughly consider if the dependency is necessary and
+an exception should be made in the `deny.toml` file.
+
+For all further configration options, please refer to the [cargo-deny documentation](https://embarkstudios.github.io/cargo-deny/).
+
+## cargo-supply-chain
+
+`cargo-supply-chain` generates reports on dependencies and their authors. These reports should be reviewed regularly.
+However, they will not cause the CI to fail.