chore: beacon endpoint docs

README.md

@@ -82,6 +82,7 @@ Open `.env` with your editor of choice
     * `full` (Full node) - A Full node contains a few recent blocks without historical states.
     * `archive` (Archive node) - An Archive node stores the complete history of the blockchain, including historical states.
 * **OP_NODE__RPC_ENDPOINT** - Specify the endpoint for the RPC of Layer 1 (e.g., Ethereum mainnet). For instance, you can use the free plan of Alchemy for the Ethereum mainnet.
+* **OP_NODE__L1_BEACON** - Specify the beacon endpoint of Layer 1. You can use [QuickNode for the beacon endpoint](https://www.quicknode.com/docs/ethereum/eth-v1-beacon-genesis).
 * **OP_NODE__RPC_TYPE** - Specify the service provider for the RPC endpoint you've chosen in the previous step. The available options are:
     * `alchemy` - Alchemy
     * `quicknode` - Quicknode (ETH only)