Add mention about restriction for types of L3 setup for simnet

[raladev]

Add mention that i cant use native/infura/external options for simnet setup

ro-ma@roma-ODROID-H2:~$ bash xud.sh --geth.mode native
1) Simnet
2) Testnet
3) Mainnet
Please choose the network: 1
Pulling image exchangeunion/utils:latest
🚀 Launching simnet environment
🌍 Checking for updates...
All up-to-date.

🏃 Warming up...


                           .___           __  .__   
          ___  _____ __  __| _/     _____/  |_|  |  
          \  \/  /  |  \/ __ |    _/ ___\   __\  |  
           >    <|  |  / /_/ |    \  \___|  | |  |__
          /__/\_ \____/\____ |     \___  >__| |____/
                \/          \/         \/           
--------------------------------------------------------------

simnet > exit
ro-ma@roma-ODROID-H2:~$ bash xud.sh --bitcoind.mode native
1) Simnet
2) Testnet
3) Mainnet
Please choose the network: 1
🚀 Launching simnet environment
🌍 Checking for updates...
All up-to-date.

🏃 Warming up...


                           .___           __  .__   
          ___  _____ __  __| _/     _____/  |_|  |  
          \  \/  /  |  \/ __ |    _/ ___\   __\  |  
           >    <|  |  / /_/ |    \  \___|  | |  |__
          /__/\_ \____/\____ |     \___  >__| |____/
                \/          \/         \/           
--------------------------------------------------------------

simnet > 

[kilrau]

Not big deal, since this only can happen with CLI parameters and sample-simnet.config doesn't contain invalid values. Will leave the issue open though, we can address it one day.