update readme (#32)

Author: leohhhn

README.md

@@ -47,9 +47,12 @@ make build
 
 To run the faucet, start the built binary:
 ```bash
-./build/faucet
+./build/faucet --mnemonic "<faucet_account_mnemonic>"
 ```
 
+The provided mnemonic will be used to derive accounts which will be used to serve
+funds to users. Make sure that the accounts derived from it are well funded.
+
 It should print something like the following. (Note the port number at the end.)
 ```
 2024-01-11T12:47:27.826+0100	INFO	cmd/logger.go:17	faucet started at [::]:8545