Update Readme.md

Add issue solution time-tracking.

Readme.md

@@ -10,6 +10,7 @@ A library to import and export various 3d-model-formats including scene-post-pro
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+[![Average time to resolve an issue](http://isitmaintained.com/badge/resolution/assimp/assimp.svg)](http://isitmaintained.com/project/assimp/assimp "Average time to resolve an issue")
 <br>
 
 APIs are provided for C and C++. There are various bindings to other languages (C#, Java, Python, Delphi, D). Assimp also runs on Android and iOS.