chore: document BROADCAST_POOL_SIZE

Author: edgurgel

README.md

@@ -190,6 +190,8 @@ If you're using the default tenant, the URL is `ws://realtime-dev.localhost:4000
 | MAX_GEN_RPC_CLIENTS                             | number  | Max amount of `gen_rpc` TCP connections per node-to-node channel                                                                                                                                                                                                                                                                |
 | REBALANCE_CHECK_INTERVAL_IN_MS                  | number  | Time in ms to check if process is in the right region                                                                                                                                                                                                                                                                           |
 | DISCONNECT_SOCKET_ON_NO_CHANNELS_INTERVAL_IN_MS | number  | Time in ms to check if a socket has no channels open and if so, disconnect it                                                                                                                                                                                                                                                   |
+| BROADCAST_POOL_SIZE                             | number  | Number of processes to relay Phoenix.PubSub messages across the cluster                                                                                                                                                                                                                                                         |
+
 
 The OpenTelemetry variables mentioned above are not an exhaustive list of all [supported environment variables](https://opentelemetry.io/docs/languages/sdk-configuration/).