Update README.md

Author: vijendra1125

README.md

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 A sample project to build a custom Mask RCNN model using Tensorflow object detection API. 
 You could find detailed documentation on usage of this repository at [my Medium blog post for Custom Mask RCNN](https://medium.com/@vijendra1125/custom-mask-rcnn-using-tensorflow-object-detection-api-101149ce0765)
 
-Tensorflow version used: 1.15.0
+Tensorflow version used: 1.13.1