Add release cadence (open-telemetry#432)

Author: trask

RELEASING.md

@@ -10,6 +10,12 @@ Every successful CI build of the main branch automatically executes `./gradlew p
 as the last step, which publishes a snapshot build to
 [Sonatype OSS snapshots repository](https://oss.sonatype.org/content/repositories/snapshots/io/opentelemetry/contrib/).
 
+## Release cadence
+
+This repository roughly targets monthly minor releases from the `main` branch on the Friday after
+the second Monday of the month (roughly a couple of days after the monthly minor release of
+[opentelemetry-java-instrumentation](https://github.com/open-telemetry/opentelemetry-java-instrumentation)).
+
 ## Preparing a new major or minor release
 
 * Close the release milestone if there is one.