chore(README): Add some badges (#9)

README.md

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 # MoLeR: A Model for Molecule Generation
 
+[![CI](https://github.com/microsoft/molecule-generation/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/microsoft/molecule-generation/actions/workflows/ci.yml)
+[![license](https://img.shields.io/github/license/microsoft/molecule-generation.svg)](https://github.com/microsoft/molecule-generation/blob/main/LICENSE)
+[![code style](https://img.shields.io/badge/code%20style-black-202020.svg)](https://github.com/ambv/black)
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 This repository contains training and inference code for the MoLeR model introduced in [Learning to Extend Molecular Scaffolds with Structural Motifs](https://arxiv.org/abs/2103.03864). We also include our implementation of CGVAE, but it currently lacks integration with the high-level model interface, and is provided mostly for reference.
 
 ## Quick start