Update README.md

Author: cocoindex-dev

README.md

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 CocoIndex is the world's first open-source engine that supports both custom transformation logic and incremental updates specialized for data indexing.
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-    <img src="https://cocoindex.io/images/cocoindex-features.png" alt="CocoIndex Features" width="600">
+    <img src="https://cocoindex.io/images/cocoindex-features.png" alt="CocoIndex Features" width="500">
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 With CocoIndex, users declare the transformation, CocoIndex creates & maintains an index, and keeps the derived index up to date based on source update, with minimal computation and changes.