Clarify usage of runtimes in async book examples

In response to discussion on Zulip
[here](https://rust-lang.zulipchat.com/#narrow/stream/187312-wg-async-foundations/topic/meeting.202021.2E02.2E11)

src/08_ecosystem/00_chapter.md

@@ -4,6 +4,10 @@ Importantly, executors, tasks, reactors, combinators, and low-level I/O futures
 are not yet provided in the standard library. In the meantime,
 community-provided async ecosystems fill in these gaps.
 
+The Async Foundations Team is interested in extending examples in the Async Book to cover multiple runtimes.
+If you're interested in contributing to this project, please reach out to us on
+[Zulip](https://rust-lang.zulipchat.com/#narrow/stream/201246-wg-async-foundations.2Fbook).
+
 ## Async Runtimes
 Async runtimes are libraries used for executing async applications.
 Runtimes usually bundle together a *reactor* with one or more *executors*.

src/09_example/01_running_async_code.md

@@ -36,9 +36,11 @@ We can't `await` or `poll` futures within synchronous code by itself.
 We'll need an asynchronous runtime to handle scheduling and running futures to completion.
 Please consult the [section on choosing a runtime](../08_ecosystem/00_chapter.md)
 for more information on asynchronous runtimes, executors, and reactors.
+Any of the runtimes listed will work for this project, but for these examples,
+we've chosen to use the `async-std` crate.
 
 ## Adding an Async Runtime
-Here, we'll use an executor from the `async-std` crate.
+The following example will demonstrate refactoring synchronous code to use an async runtime; here, `async-std`.
 The `#[async_std::main]` attribute from `async-std` allows us to write an asynchronous main function.
 To use it, enable the `attributes` feature of `async-std` in `Cargo.toml`:
 ```toml