cmd/geth: add banner for devmode on console

cmd/geth/main.go

@@ -291,6 +291,12 @@ func prepare(ctx *cli.Context) {
 
 	case ctx.GlobalIsSet(utils.DeveloperFlag.Name):
 		log.Info("Starting Geth in ephemeral dev mode...")
+		log.Warn("You are running Geth in --dev mode. Please note the following:")
+		log.Warn("\t1. This mode is only intended for fast, iterative development without assumptions on security or persistence.")
+		log.Warn("\t2. The database is created in memory unless specified otherwise. Therefore, shutting down your computer or losing power will wipe your entire block data and chain state for your dev environment.")
+		log.Warn("\t3. A random, pre-allocated developer account will be available and unlocked as eth.coinbase, which can be used for testing. The random dev account is temporary, stored on a ramdisk, and will be lost if your machine is restarted.")
+		log.Warn("\t4. Mining is enabled by default. However, the client will only seal blocks if transactions are pending in the mempool. The miner's minimum accepted gas price is 1.")
+		log.Warn("\t5. Networking is disabled; there is no listen-address, the maximum number of peers is set to 0, and discovery is disabled.")
 
 	case !ctx.GlobalIsSet(utils.NetworkIdFlag.Name):
 		log.Info("Starting Geth on Ethereum mainnet...")