add upgrade guide entry

website/content/en/highlights/2023-09-26-0-33-0-upgrade-guide.md

@@ -24,3 +24,16 @@ We cover them below to help you upgrade quickly:
 The `armv7` rpm package, `vector-<version>-1.armv7.rpm`, is now published as
 `vector-<version>-1.armv7hl.rpm` to better follow rpm guidelines. The `armv7`
 package will be no longer be published beginning in the 0.34.0 release.
+
+### Potentially impactful changes
+
+#### Async runtime default number of worker threads {#runtime-worker-threads}
+
+We've changed the default number of worker threads spawned by Vector's async runtime
+from being the number of CPUs on the host machine to the value returned by
+[`std::thread::available_parallelism()`](https://doc.rust-lang.org/stable/std/thread/fn.available_parallelism.html).
+This should be a better default value for containerized environments where the container
+has limited quotas, but note this change may impact performance.
+
+The number of worker threads used can be seen by enabling debug logging, and the value can
+be overriden by setting the `VECTOR_THREADS` environment variable.