docs: Add tracing-wasm to "Related Crates" (#841)

## Motivation

I'd like to get more feedback on our [`tracing-wasm` crate][1], as well as 
improve its discoverability.

[1]: https://crates.io/crates/tracing-wasm

## Solution

Add a link to the crate to the "Related Crates" section of the readme file.

README.md

@@ -381,6 +381,8 @@ are not maintained by the `tokio` project. These include:
  pretty printing them.
 - [`spandoc`] provides a proc macro for constructing spans from doc comments
  _inside_ of functions.
+- [`tracing-wasm`] provides a `Subscriber`/`Layer` implementation that reports 
+ events and spans via browser `console.log` and [User Timing API (`window.performance`)].
 
 (if you're the maintainer of a `tracing` ecosystem crate not in this list,
 please let us know!)
@@ -400,6 +402,8 @@ please let us know!)
 [`color-spantrace`]: https://docs.rs/color-spantrace
 [`color-eyre`]: https://docs.rs/color-eyre
 [`spandoc`]: https://docs.rs/spandoc
+[`tracing-wasm`]: https://docs.rs/tracing-wasm
+[User Timing API (`window.performance`)]: https://developer.mozilla.org/en-US/docs/Web/API/User_Timing_API
 
 **Note:** that some of the ecosystem crates are currently unreleased and
 undergoing active development. They may be less stable than `tracing` and

tracing/src/lib.rs

@@ -705,6 +705,8 @@
 //! (Linux-only).
 //! - [`tracing-bunyan-formatter`] provides a layer implementation that reports events and spans
 //! in [bunyan] format, enriched with timing information.
+//! - [`tracing-wasm`] provides a `Subscriber`/`Layer` implementation that reports
+//! events and spans via browser `console.log` and [User Timing API (`window.performance`)].
 //! - [`tide-tracing`] provides a [tide] middleware to trace all incoming requests and responses.
 //!
 //! If you're the maintainer of a `tracing` ecosystem crate not listed above,
@@ -721,6 +723,8 @@
 //! [coz]: https://github.com/plasma-umass/coz
 //! [`tracing-bunyan-formatter`]: https://crates.io/crates/tracing-bunyan-formatter
 //! [bunyan]: https://github.com/trentm/node-bunyan
+//! [`tracing-wasm`]: https://docs.rs/tracing-wasm
+//! [User Timing API (`window.performance`)]: https://developer.mozilla.org/en-US/docs/Web/API/User_Timing_API
 //! [`tide-tracing`]: https://crates.io/crates/tide-tracing
 //! [tide]: https://crates.io/crates/tide
 //!