Update README.md

Added link to older main branch.

Author: nirmay

README.md

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+## Latest Update
+In August 2025, a completely new Beckn-ONIX adapter was made available. This version introduces a Plugin framework at it's core. 
+The ONIX Adapter previous to this release is archived to a separate branch, [main-pre-plugins][https://github.com/beckn/beckn-onix/tree/main-pre-plugins] for reference.
 
 ## Overview
 
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 Built with ❤️ for the open Value Network ecosystem
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