Add note about unreleased version

README.md

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 **Hammer** is a rate-limiter for Elixir with pluggable storage backends. Hammer enables users to set limits on actions performed within specified time intervals, applying per-user or global limits on API requests, file uploads, and more.
 
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+> [!NOTE]
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+> This README is for the unreleased master branch, please reference the [official documentation on hexdocs](https://hexdocs.pm/hammer) for the latest stable release.
+
+---
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 ## Installation
 
 Hammer is [available in Hex](https://hex.pm/packages/hammer). Install by adding `:hammer` to your list of dependencies in `mix.exs`: