Add to 'async' keyword on crates.io

Author: dtolnay

Cargo.toml

@@ -8,6 +8,7 @@ description = "Type erasure for async trait methods"
 repository = "https://github.com/dtolnay/async-trait"
 documentation = "https://docs.rs/async-trait"
 readme = "README.md"
+keywords = ["async"]
 
 [lib]
 proc-macro = true