Update README.md

Include note about custom JDK version.

README.md

@@ -117,7 +117,7 @@ To submit your own implementation to 1BRC, follow these steps:
 * (Optional) If you'd like to use native binaries (GraalVM), adjust the _pom.xml_ file so that it builds that binary.
 * Create a pull request against the upstream repository, clearly stating
   * The name of your implementation class.
-  * The JDK build to use (if not specified, the latest OpenJDK 21 upstream build will be used).
+  * OpenJDK 21 is the default. If a custom JDK build is required, include the SDKMAN command `sdk use java [version]` in the launch shell script prior to application start.
   * The execution time of the program on your system and specs of the same (CPU, number of cores, RAM). This is for informative purposes only, the official runtime will be determined as described below.
 * I will run the program and determine its performance as described in the next section, and enter the result to the scoreboard.