Add benchmarks to the flatten-serde-json subcrate

flatten-serde-json/Cargo.toml

@@ -5,7 +5,12 @@ edition = "2021"
 description = "Flatten serde-json objects like elastic search"
 readme = "README.md"
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-
 [dependencies]
 serde_json = "1.0"
+
+[dev-dependencies]
+criterion = { version = "0.3", features = ["html_reports"] }
+
+[[bench]]
+name = "benchmarks"
+harness = false