Adding GH tag shield badge

Author: cliffg-softwarelibre

README.md

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 ![GH Actions Workflow Status](https://img.shields.io/github/actions/workflow/status/connectivecpp/shared-buffer/gen_docs.yml?branch=main&label=GH%20Actions%20generate%20docs)
 
+![GH Tag](https://img.shields.io/github/v/tag/connectivecpp/shared-buffer?label=GH%20tag)
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 ## Overview
 
 The `shared_buffer` classes are reference counted `std::byte` buffer classes useful for asynchronous networking. In particular, the Asio asynchronous networking library requires a buffer to be kept alive and valid until the outstanding IO operation (e.g. a network write) is completed. A straightforward and idiomatic way to achieve this is by using reference counted buffers.