Add SSWG badge

Author: adam-fowler

README.md

@@ -1,5 +1,6 @@
 # MQTT NIO 
 
+[![sswg:sandbox|94x20](https://img.shields.io/badge/sswg-sandbox-lightgrey.svg)](https://github.com/swift-server/sswg/blob/master/process/incubation.md#sandbox-level)
 [<img src="http://img.shields.io/badge/swift-5.5-brightgreen.svg" alt="Swift 5.5" />](https://swift.org)
 [<img src="https://github.com/adam-fowler/mqtt-nio/workflows/CI/badge.svg" />](https://github.com/adam-fowler/mqtt-nio/workflows/CI/badge.svg)