docs: document bors

Author: marcoieni

service-catalog/bors/README.md

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+# Bors
+
+The bors [module](https://github.com/rust-lang/simpleinfra/tree/master/terragrunt/modules/bors)
+contains the infrastructure of [bors](https://github.com/rust-lang/bors), the Rust rewrite of
+[homu](https://github.com/rust-lang/homu).
+
+`Homu` is deployed in the legacy account, while `bors` is deployed in the
+`bors-staging` and `bors-prod` account, depending on the environment.
+
+Bors is deployed as a [Fargate](https://aws.amazon.com/fargate/) service
+([ECS](https://aws.amazon.com/ecs/)) in the `us-east-2` region.
+Bors uses an RDS PostgreSQL database.
+
+To deploy a new version, the
+[deployment](https://github.com/rust-lang/bors/blob/main/.github/workflows/deploy.yml)
+pipeline:
+
+1. Pushes the Docker image to the [ECR](https://aws.amazon.com/ecr/) repository.
+2. Updates the ECS service, forcing a new deployment.
+
+## Versions
+
+These are the versions we need to keep up-to-date:
+
+- Operating system: Ubuntu. Version specified in the
+  [Dockerfile]
+- Rust toolchain: specified in the [Dockerfile]
+- Rust dependencies: specified in the
+  [Cargo.toml](https://github.com/rust-lang/bors/blob/main/Cargo.toml)
+- PostgreSQL: version specified in
+  [Terraform](https://github.com/rust-lang/simpleinfra/blob/master/terragrunt/modules/bors/main.tf)
+
+No automation is in place as of August 2024.
+
+[Dockerfile]: https://github.com/rust-lang/bors/blob/main/Dockerfile