chore: adds README

tools/fetcher/README.md

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+# fetcher
+
+> A simple CLI for fetching Jormungandr archives and artifacts
+
+The `fetcher` CLI provides a simple interface for fetching Jormungandr archives and artifacts from a configured object store.
+By default, it uses AWS S3.
+The primary use of this CLI is in CI pipelines or container entrypoint scripts where archives or artifacts are needed.
+By specifying the appropriate identification parameters, the CLI will automatically download the respective files from an object
+store.
+
+## Usage
+
+The CLI uses cloud SDKs (like AWS) for authentication.
+It assumes you've configured the appropriate credentials in your environment to authenticate with the cloud provider.
+
+### Artifacts
+
+To fetch a specific Jormungandr genesis file:
+
+```
+# Fetch v1.0.0 of the genesis file from the fund10 dev environment
+$ fetcher --bucket "artifacts-bucket" artifact -e "dev" -f "fund10" -t "genesis" -v "1.0.0" ./block0.bin
+```
+
+To fetch a specific vit-ss database file:
+
+```
+# Fetch v1.0.0 of the vit-ss database from the fund10 dev environment
+$ fetcher --bucket "artifacts-bucket" artifact -e "dev" -f "fund10" -t "vit" -v "1.0.0" ./block0.bin
+```
+
+In either case, you can omit the version (`-v`) flag and the CLI will download the "default" (unversioned) artifact.
+This is offered to support backwards compatability when using unversioned artifacts.
+
+### Archives
+
+To fetch a specific Jormungandr archive:
+
+```
+$ fetcher --bucket "archives-bucket" archive -e "dev" -i "f41c4470-03dd-4d3f-a325-6f784259b9c5" ./artifact.tar.zstd
+```
+
+Artifact IDs can be found using the archiver API (i.e., https://archiver.dev.projectcatalyst.io/api/v1/archives/).