Document reliance on resolver's version 2

Cargo.toml

@@ -2,7 +2,11 @@
 
 # Was necessary after switching to dev tree-sitter to fix this warning:
 # > some crates are on edition 2021 which defaults to `resolver = "2"`, but
-# > virtual workspaces default to `resolver = "1"`
+# > virtual workspaces default to `resolver = "1"`.
+#
+# Also necessary to enable the `testing` feature of harp only when testing
+# (i.e. when building downstream packages like Ark with Harp's `testing`
+# feature set in `dev-dependencies`).
 resolver = "2"
 
 members = [