Add note to not update toml.

Author: ehuss

Cargo.toml

@@ -33,7 +33,7 @@ serde = { version = "1.0.163", features = ["derive"] }
 serde_json = "1.0.96"
 shlex = "1.1.0"
 tempfile = "3.4.0"
-toml = "0.5.11"
+toml = "0.5.11" # Do not update, see https://github.com/rust-lang/mdBook/issues/2037
 topological-sort = "0.2.2"
 
 # Watch feature