Add badges to README

README.md

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+| Swift 5.3 |
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+|[![Amazon Linux 2](https://github.com/dduan/Termbox/workflows/Amazon%20Linux%202/badge.svg)](https://github.com/dduan/Termbox/actions?query=workflow%3A%22Amazon+Linux+2%22)|
+|[![CentOS 8](https://github.com/dduan/Termbox/workflows/CentOS%208/badge.svg)](https://github.com/dduan/Termbox/actions?query=workflow%3A%22CentOS+8%22)|
+|[![macOS 11.15](https://github.com/dduan/Termbox/workflows/macOS%2011.15/badge.svg)](https://github.com/dduan/Termbox/actions?query=workflow%3A%22macOS+11.15%22)|
+|[![Ubuntu Bionic](https://github.com/dduan/Termbox/workflows/Ubuntu%20Bionic/badge.svg)](https://github.com/dduan/Termbox/actions?query=workflow%3A%22Ubuntu+Bionic%22)|
+|[![Ubuntu Focal](https://github.com/dduan/Termbox/workflows/Ubuntu%20Focal/badge.svg)](https://github.com/dduan/Termbox/actions?query=workflow%3A%22Ubuntu+Focal%22)|
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 This is a Swift wrapper for [termbox](https://github.com/nsf/termbox).
 
 Both the original C library and a Swift wrapper are included for Swift Package