clarify startup benchmark (#3019)

benchmark/sirun/startup/README.md

@@ -1,3 +1,7 @@
 This is a simple startup test. It tests with an without the tracer, and with
 and without requiring every dependency and devDependency in the package.json,
 for a total of four variants.
+
+While it's unrealistic to load all the tracer's devDependencies, the intention
+is to simulate loading a lot of dependencies for an application, and have them
+either be intercepted by our loader hooks, or not.