Update README.md

Author: 喵喵大人

README.md

@@ -7,6 +7,10 @@
 <a href="https://codecov.io/gh/CatLib/Core">
   <img src="https://codecov.io/gh/CatLib/Core/branch/master/graph/badge.svg" alt="Codecov" />
 </a>
+<a href="https://github.com/CatLib/Core/releases">
+  <img src="https://img.shields.io/nuget/dt/CatLib.Core.svg" alt="Downloads" />
+</a>
+</p>
 
 > We are currently preparing to convert the comments in the code to the English version. We look forward to your joining.