Update README.md (#1330)

Autogenerated changes after running the `sbt docs/generateReadme`
command of the [zio-sbt-website](https://zio.dev/zio-sbt) plugin.

I will automatically update the README.md file whenever there is new
change for README.md, e.g.
- After each release, I will update the version in the installation
section.
- After any changes to the "docs/index.md" file, I will update the
README.md file accordingly.

Co-authored-by: ZIO Assistant <zio-assistant[bot]@users.noreply.github.com>

README.md

@@ -26,6 +26,12 @@ libraryDependencies += "dev.zio" %% "zio-kafka-testkit" % "2.8.2" % Test
 Snapshots are available on Sonatype's snapshot repository https://oss.sonatype.org/content/repositories/snapshots.
 [Browse here](https://oss.sonatype.org/content/repositories/snapshots/dev/zio/zio-kafka_3/) to find available versions.
 
+For `zio-kafka-testkit` together with Scala 3, you also need to add the following to your `build.sbt` file:
+
+```scala
+excludeDependencies += "org.scala-lang.modules" % "scala-collection-compat_2.13"
+```
+
 ## Example
 
 Let's write a simple Kafka producer and consumer using ZIO Kafka with ZIO Streams. Before everything, we need a running instance of Kafka. We can do that by saving the following docker-compose script in the `docker-compose.yml` file and run `docker-compose up`: