add comment to env example

.env.example

@@ -4,12 +4,19 @@
 # RPCs
 # ############################
 
-L1_CHAIN_ID=10
-L2_CHAIN_ID=11155420
+# ID of the L1 chain. For example, 1 is the ID of the Ethereum Mainnet.
+L1_CHAIN_ID=1
 
-L1_PRC_URL=https://sepolia.infura.io/v3/
-L2_PRC_URL=https://optimism-sepolia.infura.io/v3/
+# ID of the L2 chain. For example, 1 is the ID of the Seneium Mainnet.
+L2_CHAIN_ID=1868
 
+# URL of the L1 RPC. For example, https://sepolia.infura.io/v3/ is the URL of the Sepolia testnet.
+L1_PRC_URL=https://sepolia.infura.io/v3/<rpc_key>
+
+# URL of the L2 RPC. For example, https://optimism-sepolia.infura.io/v3/ is the URL of the Optimism testnet.
+L2_PRC_URL=https://optimism-sepolia.infura.io/v3/<rpc_key>
+
+# Address of the rollup cross-domain messenger. Can be found in rollup docs.
 L1_CROSSDOMAIN_MESSENGER=0x25ace71c97B33Cc4729CF772ae268934F7ab5fA1
 L2_CROSSDOMAIN_MESSENGER=0x4200000000000000000000000000000000000007