crates_io_smoke_test: Add README.md

Author: Turbo87

crates/crates_io_smoke_test/README.md

@@ -0,0 +1,11 @@
+# crates_io_smoke_test
+
+This package contains a basic smoke test for the <https://staging.crates.io>
+environment. It uses the API to fetch the metadata of a test crate, publishes
+a new patch version, attempts to download the crate file, and checks that the
+git and sparse indexes contain the new version.
+
+Note that a valid `CARGO_REGISTRY_TOKEN` environment variable is required to
+run the smoke test. This token must have the `publish` permission for the test
+crate. If `--skip-publish` is passed, the smoke test will not publish a new
+version of the test crate.

crates/crates_io_smoke_test/src/main.rs

@@ -1,3 +1,5 @@
+#![doc = include_str!("../README.md")]
+
 mod api;
 mod cargo;
 mod exit_status_ext;