doc: mark readline promise implementation as experimental

aduh95 · aduh95 · commit 90290d14cfcc · 2021-09-25T00:19:54.000+02:00
Refs: nodejs#37947 (comment)
diff --git a/doc/api/readline.md b/doc/api/readline.md
@@ -535,6 +535,8 @@ line prompts are included in the calculations.
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+> Stability: 1 - Experimental
+
 ### Class: `readlinePromises.Interface`
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