doc: comment the bigdecimal down bump

Author: mzabaluev

Cargo.toml

@@ -484,6 +484,7 @@ backtrace = "0.3.58"
 bcs = { git = "https://github.com/movementlabsxyz/bcs.git", rev = "bc16d2d39cabafaabd76173dd1b04b2aa170cf0c" }
 better_any = "0.1.1"
 bellman = { version = "0.13.1", default-features = false }
+# Bumped down to 0.3 due to https://github.com/diesel-rs/diesel/issues/4477
 bigdecimal = { version = "0.3", features = ["serde"] }
 version-compare = "0.1.1"
 bitvec = "1.0.1"