Add example in README for mining in a desired time

README.md

@@ -58,6 +58,9 @@ Options:
 
 -   (optional) `rpc_url` Node RPC url e.g. http://127.0.0.1:8545
 -   (optional) `chain_id` Chain id e.g. 636363
+-   (optional) `desired_vdf_duration` Desired VDF duration = mining duration e.g. 60secs
+
+Example: `cargo run mine http://127.0.0.1:8545' 636363 60`
 
 ## Env vars