Add comment on tokio library versions

Cargo.toml

@@ -283,6 +283,10 @@ tiny-keccak = { version = "2.0.2", features = ["keccak", "sha3"] }
 tracing = "0.1.34"
 tracing-subscriber = { version = "0.3.17", features = ["json", "env-filter"] }
 trybuild = "1.0.70"
+# NOTE: The goal is to have the tokio dependencies (which are mostly built from the same repo)
+# have latest and consistent revisions. The selected revisions should be checked to verify
+# that transitive dependencies haven't regressed versions. When the tokio project publishes
+# new minor versions, the specifiers below will need to be updated accordingly.
 tokio = { version = "1", features = ["full"] }
 tokio-io-timeout = "1"
 tokio-metrics = "0.3"