Add a section about how to capture wgpu spans when profiling

docs/profiling.md

@@ -2,7 +2,11 @@
 
 ## Runtime Flame Graph: `tracing` spans
 
-Bevy has built-in [tracing](https://github.com/tokio-rs/tracing) spans to make it cheap and easy to profile Bevy ECS systems, render logic, engine iternals, and user app code. Enable the `trace` cargo feature to enable Bevy's built-in spans. You also need to select a `tracing` backend using the following cargo features:
+Bevy has built-in [tracing](https://github.com/tokio-rs/tracing) spans to make it cheap and easy to profile Bevy ECS systems, render logic, engine iternals, and user app code. Enable the `trace` cargo feature to enable Bevy's built-in spans.
+
+If you also want to include `wgpu` tracing spans when profiling, they are emitted at the `tracing` `info` level so you will need to make sure they are not filtered out by the `LogSettings` resource's `filter` member which defaults to `wgpu=error`. You can do this by setting the `RUST_LOG=info` environment variable when running your application.
+
+You also need to select a `tracing` backend using the following cargo features:
 
 ### Backend: trace_chrome