Add status badge to readme

Author: amayers

README.md

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 # NumericText
 
+![Build](https://github.com/amayers/NumericText/workflows/Swift/badge.svg)
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 A simple SwiftUI `TextField` that limits user input to numbers.
 
 The most common response in https://stackoverflow.com/questions/58733003/swiftui-how-to-create-textfield-that-only-accepts-numbers (the top search result), advocates just setting a numeric keyboard. But that totally misses cases when used with hardware keyboards. Many of the suggested