docs: document rust ci

service-catalog/rust-ci/README.md

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+# Rust CI
+
+The Infrastructure team maintains the [rust-lang/rust] Continuous Integration, which lives in the [workflows] directory.
+
+The [terraform/rust-ci] module provisions some resources used by the CI:
+- the `rust-lang-ci-sccache2` S3 bucket, used to store [sscache] compilation artifacts to speed up the rustc compile time. Served by Cloudfront CDN at `ci-caches.rust-lang.org`.
+- the `rust-lang-ci2` S3 bucket, used to store compilation artifacts. Served by Cloudfront CDN at `ci-artifacts.rust-lang.org`.
+
+[sscache]: https://github.com/mozilla/sccache
+[terraform/rust-ci]: https://github.com/rust-lang/simpleinfra/tree/master/terraform/rustc-ci
+[rust-lang/rust]: https://github.com/rust-lang/rust/
+[workflows]: https://github.com/rust-lang/rust/tree/master/.github/workflows