docs: document rust playground

service-catalog/playground/README.md

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+# Rust Playground
+
+The [Rust Playground] is a service that allows users to run Rust code in a web
+browser. More info in the Playground [help page].
+
+[terraform/playground] deploys the Rust Playground infrastructure.
+
+The Playground is deployed in an AWS EC2. The [deployment]
+directory contains information and scripts on how to deploy it.
+
+## Versions
+
+- Ubuntu version of the EC2 instance.
+- [package.json](https://github.com/rust-lang/rust-playground/blob/main/ui/frontend/package.json).
+- [Cargo.lock](https://github.com/rust-lang/rust-playground/blob/main/ui/Cargo.lock).
+
+[Rust Playground]: https://play.rust-lang.org/
+[help page]: https://play.rust-lang.org/help
+[terraform/playground]: https://github.com/rust-lang/simpleinfra/tree/master/terraform/playground
+[deployment]: https://github.com/rust-lang/rust-playground/tree/main/deployment