chore(README): add announcements (#1399)

update readme for announcements

README.md

@@ -6,6 +6,14 @@
 MQTT.js is a client library for the [MQTT](http://mqtt.org/) protocol, written
 in JavaScript for node.js and the browser.
 
+| :warning: WARNING          |
+|:---------------------------|
+| our default branch has been moved from *master* to *main*      |
+
+| :boom: MQTT.js vNext          |
+|:---------------------------|
+| Want to contribute to the next generation of **MQTT.js**? Join the discussion [here](https://github.com/mqttjs/MQTT.js/discussions/1324).       |
+
 ## Table of Contents
 * [__MQTT.js vNext__](#vnext)
 * [Upgrade notes](#notes)