Add backlink to V1

README.md

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 This work stands on the shoulders of 2 robust researches: [Naver's From Distillation to Hard Negative Sampling: Making Sparse Neural IR Models More Effective paper](https://arxiv.org/pdf/2205.04733.pdf) and [Google's SparseEmbed](https://storage.googleapis.com/gweb-research2023-media/pubtools/pdf/79f16d3b3b948706d191a7fe6dd02abe516f5564.pdf). 
 Props to both the teams for such a robust work.
 
+**This is a 2nd iteration in this series. Try V1 here:** [prithivida/Splade_PP_en_v1](https://huggingface.co/prithivida/Splade_PP_en_v1)
+
 ## 1. What are Sparse Representations and Why learn one?
 
 **Beginner ?** expand this. **Expert in Sparse & Dense representations ?** feel free skip to next section 2,