Update readme

Author: Stjepan Glavina

README.md

@@ -7,10 +7,13 @@ https://github.com/async-rs/async-task)
 [![Documentation](https://docs.rs/async-task/badge.svg)](https://docs.rs/async-task)
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-A task abstraction for building executors.
+Task abstraction for building executors.
 
-This crate makes it possible to build an efficient and extendable executor in few lines of
-code.
+To spawn a future onto an executor, we first need to allocate it on the heap and keep some
+state alongside it. The state indicates whether the future is ready for polling, waiting to be
+woken up, or completed. Such a future is called a *task*.
+
+This crate helps with task allocation and polling its future to completion.
 
 ## License