Add Codecov badge to `README.md` [ci skip]

README.md

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 *A lightweight [platform-accelerated](https://en.wikipedia.org/wiki/Single_instruction,_multiple_data) library for [biological motif](https://en.wikipedia.org/wiki/Sequence_motif) scanning using [position weight matrices](https://en.wikipedia.org/wiki/Position_weight_matrix)*.
 
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