Add a disclaimer saying this is a basic executor (#74)

In many issues I've mentioned that the executors in this crate are just
reference executors. However, this is not documented in the crate
itself.

This commit adds a disclaimer to the crate documentation and to
README.md that these are reference executors that shouldn't be relied on
for performance.

Signed-off-by: John Nunley <dev@notgull.net>

README.md

@@ -11,6 +11,13 @@ https://docs.rs/async-executor)
 
 Async executors.
 
+This crate provides two reference executors that trade performance for
+functionality. They should be considered reference executors that are "good
+enough" for most use cases. For more specialized use cases, consider writing
+your own executor on top of [`async-task`].
+
+[`async-task`]: https://crates.io/crates/async-task
+
 ## Examples
 
 ```rust

src/lib.rs

@@ -1,5 +1,12 @@
 //! Async executors.
 //!
+//! This crate provides two reference executors that trade performance for
+//! functionality. They should be considered reference executors that are "good
+//! enough" for most use cases. For more specialized use cases, consider writing
+//! your own executor on top of [`async-task`].
+//!
+//! [`async-task`]: https://crates.io/crates/async-task
+//!
 //! # Examples
 //!
 //! ```