Add LGTM badges to README

Author: Irina Nicolae

README.md

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 # Adversarial Robustness Toolbox (ART v0.3.0)
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 This is a library dedicated to **adversarial machine learning**. Its purpose is to allow rapid crafting and analysis of attacks and defense methods for machine learning models. The Adversarial Robustness Toolbox provides an implementation for many state-of-the-art methods for attacking and defending classifiers.