chore(v1): add warning

Author: natemoo-re

README.md

@@ -12,6 +12,9 @@
 <h4 align="center"><a href="packages/core#readme"><code>@clack/core</code></a>: unstyled, extensible primitives for CLIs</h4>
 <h4 align="center"><a href="packages/prompts#readme"><code>@clack/prompts</code></a>: beautiful, ready-to-use CLI prompt components</h4>
 
+> [!WARNING]
+> Clack's `main` branch is tracking the `alpha` release line for `v1.0.0+`. To view the relatively stable `v0` line, please browse the [v0](https://github.com/bombshell-dev/clack/tree/v0) branch.
+
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