chore: added arbitrum rinkeby rpc to .env.example

Author: jaybuidl

.env.example

@@ -36,7 +36,7 @@ REACT_APP_WALLETCONNECT_BRIDGE_URL=https://bridge.walletconnect.org
 REACT_APP_NOTIFICATIONS_API_URL=http://localhost:3001
 # JSON string mapping networkID to JSON RPC url.
 # Example: REACT_APP_RPC_URLS={"1":"https://mainnet.infura.io/v3/1337deadbeef...", "42":"https://kovan.infura.io/v3/1337deadbeef..."}
-REACT_APP_RPC_URLS={"100":"https://rpc.xdaichain.com/","1":"https://mainnet.infura.io/v3/","42":"https://kovan.infura.io/v3/","4":"https://rinkeby.infura.io/v3/"}
+REACT_APP_RPC_URLS={"100":"https://rpc.xdaichain.com/","1":"https://mainnet.infura.io/v3/","42":"https://kovan.infura.io/v3/","4":"https://rinkeby.infura.io/v3/","421611":"https://rinkeby.arbitrum.io/rpc"} 
 
 # JSON string mapping networkID to Fortmatic api key.
 # Example: REACT_APP_FORMATIC_API_KEYS={"1":"pk_live_FCEE7548E5...", "42":"pk_live_0916382..."}