chore: Document current MSRV (bounded by handlebars 6.3's MSRV)

Author: alerque

Cargo.toml

@@ -3,6 +3,7 @@ name = "handlebars-fluent"
 version = "0.3.2"
 authors = ["Manish Goregaokar <[email protected]>"]
 edition = "2018"
+rust-version = "1.73.0"
 description = "Handlebars helpers for the Fluent internationalization framework"
 license = "MIT/Apache-2.0"
 documentation = "https://docs.rs/handlebars-fluent/"