add 'new in LightGBM v4.0' talk

whats-new-in-lightgbm-v4/README.md

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+# What's new in LightGBM v4.0
+
+## Description
+
+> In this talk, attendees will learn about recent developments in LightGBM, a popular open source gradient boosting library, leading to its v4.0.0 release.
+
+> The talk will describe in detail a few recent additions to the library, including:
+> * CUDA-based GPU acceleration
+> * new options to make installation of a GPU-enabled Python package easier
+> * interoperability improvements for the R and Python package
+> * algorithmic improvements
+
+## Where this talk has been given:
+
+* (virtual) Mercado Libre SyncTech talk series, September 2023 ([slides](https://docs.google.com/presentation/d/1V_OIsd2-lI3OX4oLvYVttlJa_vPTe_wF7oDIvqkVfuA/edit?usp=sharing)
+
+### Useful Links
+
+* release notes: https://github.com/microsoft/LightGBM/releases/tag/v4.0.0