Add exercise async1

The goal here was to get the first bit of "muscle memory" for using the
async and await keywords. The little story should make it more intuitive
for users why asynchronous programming is needed in the first place.

This exercise will be moved to the location corresponding to the book in
a later commit, to keep the diff of this one clean.

exercises/24_async/README.md

@@ -0,0 +1,13 @@
+# Async
+
+Asynchronous programming is a model where tasks are delegated to a runtime that executes them concurrently.
+It is particularly efficient for applications where many independent IO-operations are performed, e.g. web servers.
+
+Rust provides the necessary primitives to do asynchronous programming in the language.
+However, Rust's standard library does not include a runtime.
+For these exercises, we will use the popular runtime called `tokio`.
+
+## Further information
+
+- [Fundamentals of Asynchronous Programming](https://doc.rust-lang.org/book/ch17-00-async-await.html)
+- [Tokio documentation](https://docs.rs/tokio/latest/tokio/)