Add state module docs

src/state.rs

@@ -1,3 +1,23 @@
+//! Application state at runtime, including the database connection and other shared resources.
+//! 
+//! Currently, it only contains the database connection, but it can be extended in the future to
+//! include other resources that need to be shared across the application, such as caches or
+//! configuration settings.
+//! 
+//! It is shared around the application using an `Arc`, allowing it to be cloned and passed around
+//! without needing to worry about ownership or lifetimes. This is particularly useful in an
+//! asynchronous context.
+//! 
+//! ```
+//! let state = State {
+//!     db: database::init("127.0.0.1:9042").await?,
+//! };
+//! 
+//! fn handler_which_needs_state(state: Arc<State>) {
+//!     // Use the state here
+//! }
+//! ```
+
 use scylla::client::session::Session;
 
 pub struct State {