Update SSWG Graduation Level (#409)

Author: fabianfett

README.md

@@ -6,7 +6,7 @@
 </picture>
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 <br>
-<a name=""><img src="https://img.shields.io/badge/sswg-incubating-green.svg" alt="SSWG Incubation"></a>
+<a name="https://www.swift.org/sswg/incubation-process.html"><img src="https://img.shields.io/badge/sswg-graduated-green.svg" alt="SSWG Incubation Level: Graduated"></a>
 <a href="https://api.vapor.codes/postgresnio/documentation/postgresnio/"><img src="https://img.shields.io/badge/read_the-docs-2196f3.svg" alt="Documentation"></a>
 <a href="LICENSE"><img src="https://img.shields.io/badge/license-MIT-brightgreen.svg" alt="MIT License"></a>
 <a href="https://github.com/vapor/postgres-nio/actions/workflows/test.yml"><img src="https://github.com/vapor/postgres-nio/actions/workflows/test.yml/badge.svg" alt="Continuous Integration"></a>