CONTRIBUTING: Add benchmarking lines (#3930)

* CONTRIBUTING: Add benchmarking lines

Co-authored-by: Joseph Woodward <joseph.woodward@redpanda.com>

Author: twmb

CONTRIBUTING.md

@@ -27,6 +27,8 @@ To certify a connector, it must meet the following requirements:
 - **1.3.1** Code is idiomatic following Effective Go recommendations, is readable, and is consistent with the broader Redpanda Connect code base.  
 - **1.3.2** Tests should cover end-to-end functionality and prove that the connector works across supported configurations.  
 - **1.3.3** Integration tests verify core workflows and are runnable in CI.
+- **1.3.4** Benchmarks have been run at various throughput levels so that we can determine CPU and memory trendlines based on usage.
+- **1.3.5** If a corresponding Kafka Connect connector exists, benchmarks have been run against it so we can compare it against our throughput and ensure Redpanda Connect's is comparable or better.
 
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