Add metadata to benchmarking module. (#163)

benchmarking/Cargo.toml

 [package]
 name = "orml-benchmarking"
+description = "Provide macro to benchmark Substrate runtime."
+repository = "https://github.com/open-web3-stack/open-runtime-module-library/tree/master/benchmarking"
+license-file = "../LICENSE"
 version = "0.0.1"
 authors = ["Laminar Developers <hello@laminar.one>"]
 edition = "2018"