crates_io_session: Add README.md

Author: Turbo87

crates/crates_io_session/README.md

@@ -0,0 +1,7 @@
+# crates_io_session
+
+This package contains a `SessionExtension` extractor for the
+[`axum`](https://docs.rs/axum) web framework and a corresponding
+`attach_session()` middleware based on a signed `cargo_session` cookie. 
+This abstraction allows us to save and retrieve data from the session
+cookie in a safe way.

crates/crates_io_session/src/lib.rs

@@ -1,3 +1,5 @@
+#![doc = include_str!("../README.md")]
+
 use axum::extract::{Extension, FromRequestParts, Request};
 use axum::middleware::Next;
 use axum::response::{IntoResponse, Response};