docs: add current developer feature index

docs/benchmarks/README.md

@@ -1,5 +1,9 @@
 # Simulation benchmark ledger
 
+The [testing and benchmarks guide](../FEATURE_TESTING_AND_BENCHMARKS.md)
+explains how to add a workload, what each current result proves, and which
+rendered-performance claims are intentionally still unmeasured.
+
 This directory stores reviewed benchmark snapshots. Each snapshot must name the
 workload, seed, Godot version, hardware context, sample count, simulated tick
 count, and important exclusions so later comparisons remain honest.