Update README.md

Author: lemire

README.md

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 SIMDCompressionAndIntersection
 ======================
 [![Build Status](https://travis-ci.org/lemire/SIMDCompressionAndIntersection.png)](https://travis-ci.org/lemire/SIMDCompressionAndIntersection)
+[![Code Quality: Cpp](https://img.shields.io/lgtm/grade/cpp/g/lemire/SIMDCompressionAndIntersection.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/lemire/SIMDCompressionAndIntersection/context:cpp)
+
 
 As the name suggests, this is a C/C++ library for fast
 compression and intersection of lists of sorted integers using