Added build status badge

Author: synercoder

README.md

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 Primitives with types attached (think mm, cm, in, px) that can be used for 2D graphics.
 
+## Build status
+
+Develop: ![.NET Core](https://github.com/synercoder/Primitives/workflows/.NET%20Core/badge.svg?branch=develop)
+
 ## License
 This project is licensed under MIT license.