prepare rt v2.0.0-beta.3

actix-rt/src/lib.rs

@@ -1,4 +1,37 @@
 //! Tokio-based single-thread async runtime for the Actix ecosystem.
+//!
+//! In most parts of the the Actix ecosystem, it has been chosen to use !Send futures. For this
+//! reason, a single-threaded runtime is appropriate since it is guaranteed that futures will not
+//! be moved between threads. This can result in small performance improvements over cases where
+//! atomics would otherwise be needed.
+//!
+//! To achieve similar performance to multi-threaded, work-stealing runtimes, applications
+//! using `actix-rt` will create multiple, mostly disconnected, single-threaded runtimes.
+//! This approach has good performance characteristics for workloads where the majority of tasks
+//! have similar runtime expense.
+//!
+//! The disadvantage is that idle threads will not steal work from very busy, stuck or otherwise
+//! backlogged threads. Tasks that are disproportionately expensive should be offloaded to the
+//! blocking thread-pool using [`task::spawn_blocking`].
+//!
+//! # Examples
+//! ```
+//! use std::sync::mpsc;
+//! use actix_rt::{Arbiter, System};
+//!
+//! let _ = System::new();
+//!
+//! let (tx, rx) = mpsc::channel::<u32>();
+//!
+//! let arbiter = Arbiter::new();
+//! arbiter.spawn_fn(move || tx.send(42).unwrap());
+//!
+//! let num = rx.recv().unwrap();
+//! assert_eq!(num, 42);
+//!
+//! arbiter.stop();
+//! arbiter.join().unwrap();
+//! ```
 
 #![deny(rust_2018_idioms, nonstandard_style)]
 #![allow(clippy::type_complexity)]