Add `streamAllMessages()` snippet to readme

Based on the conversation in https://discord.com/channels/831836269558235136/993229186951028796/1039310518348951653, I think it would be a good idea to add a snippet to the readme that illustrates use of streamAllMessages(). I'm thinking the easiest way would be to update the instructions at https://xmtp.org/docs/client-sdk/javascript/tutorials/quickstart#listen-for-new-messages-in-a-conversation, adding at the bottom:

To listen for any incoming messages from all conversations, use streamAllMessages():

for await (const message of await xmtp.conversations.streamAllMessages()) {

 if (message.senderAddress === xmtp.address) {
   // This message was sent from me
   continue
 }
 console.log(`New message from ${message.senderAddress}: ${message.content}`)
}

I'd be happy to take this on if you want to assign it to me.

xmtp / xmtp-js

Add `streamAllMessages()` snippet to readme #214