Add unstable comment.

ChenPi11 · ChenPi11 · commit 6f8a1ba1f9e8 · 2025-02-10T23:44:35.000+08:00
diff --git a/docs/README.md b/docs/README.md
@@ -4,6 +4,8 @@
 
 These docs is for `cppp-reiconv 3.0.0`.
 
+It's a unstable version.
+
 cppp-reiconv is a portable C/C++ library for converting between character
 encodings and locale charset detection.
 
