mention why we pin unicode-width (#972)

data-url/Cargo.toml

@@ -17,6 +17,7 @@ alloc = []
 
 [dev-dependencies]
 tester = "0.9"
+# We pin this transitive dev dep so that MSRV CI can continue to run.
 unicode-width = "=0.1.12"
 serde = {version = "1.0", features = ["derive"]}
 serde_json = "1.0"