Add SSWG sandbox badge.

Author: mattpolzin

README.md

@@ -1,4 +1,8 @@
-[![MIT license](http://img.shields.io/badge/license-MIT-lightgrey.svg)](http://opensource.org/licenses/MIT) [![Swift 5.1](http://img.shields.io/badge/Swift-5.1-blue.svg)](https://swift.org) [![Build Status](https://app.bitrise.io/app/2f7379e33723d853/status.svg?token=Jx4X3su3oE59z_rJBRC_og&branch=master)](https://app.bitrise.io/app/2f7379e33723d853)
+[![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) [![Swift 5.1+](http://img.shields.io/badge/Swift-5.1-blue.svg)](https://swift.org)
+
+[![MIT license](http://img.shields.io/badge/license-MIT-lightgrey.svg)](http://opensource.org/licenses/MIT) 
+
+[![Build Status](https://app.bitrise.io/app/2f7379e33723d853/status.svg?token=Jx4X3su3oE59z_rJBRC_og&branch=master)](https://app.bitrise.io/app/2f7379e33723d853)
 
 # OpenAPIKit