[DOC] add references

docs/References.rst

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 References
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+Zhi-Hua Zhou. Abductive learning: `Towards bridging machine learning and logical reasoning. <https://link.springer.com/epdf/10.1007/s11432-018-9801-4?author_access_token=jgJe1Ox3Mk-K7ORSnX7jtfe4RwlQNchNByi7wbcMAY7_PxTx-xNLP7Lp0mIZ04ORp3VG4wioIBHSCIAO3B_TBJkj87YzapmdnYVSQvgBIO3aEpQWppxZG25KolINetygc2W_Cj2gtoBdiG_J1hU3pA==>`_. **Science China Information Sciences**, 2019, 62: 076101.
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+Zhi-Hua Zhou and Yu-Xuan Huang. `Abductive learning <https://www.lamda.nju.edu.cn/publication/chap_ABL.pdf>`_. In P. Hitzler and M. K. Sarker eds., **Neuro-Symbolic Artificial Intelligence: The State of the Art**, IOP Press, Amsterdam, 2022, p.353-379
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-.. contents:: Table of Contents