docs: add extend

Author: mkg20001

README.md

@@ -540,6 +540,7 @@ You can see the bundle in action in the [custom libp2p example](examples/custom-
 - `overrideFunction` (object):
     - A function that allows overriding parts of libp2p's config, using dynamic arguments, without fully replacing the bundle as a custom libp2p bundle would do
     - It is called with the arguments `{ datastore, peerInfo, peerBook, options, config }`
+- `extend` (boolean): Whether to fully replace the transport/discover arrays (`false`) or to concat them (`true`)
 - `modules` (object):
     - `transport` (Array<[libp2p.Transport](https://github.com/libp2p/interface-transport)>): An array of Libp2p transport classes/instances to use _instead_ of the defaults. See [libp2p/interface-transport](https://github.com/libp2p/interface-transport) for details.
     - `peerDiscovery` (Array<[libp2p.PeerDiscovery](https://github.com/libp2p/interface-peer-discovery)>): An array of Libp2p peer discovery classes/instances to use _instead_ of the defaults. See [libp2p/peer-discovery](https://github.com/libp2p/interface-peer-discovery) for details. If passing a class, configuration can be passed using the config section below under the key corresponding to you module's unique `tag` (a static property on the class)