Adding README.md to Describe the SMILES String Kernel

gauche/kernels/string_kernels/README.md

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+It should be noted that the SMILES string kernel from [1] is implemented in the notebooks as "bag-of-SMILES" and "bag-of-amino-acids" for molecules and proteins respectively.
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+__**References**__
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+[1] D-S Cao, J-C Zhao, Y-N Yang, C-X Zhao, J Yan, S Liu, Q-N Hu, Q-S Xu, and Y-Z Liang. [In silico toxicity prediction by support vector machine and SMILES representation-based string kernel](https://pubmed.ncbi.nlm.nih.gov/22224501/). SAR and QSAR in Environmental Research, 2012.