Added the versionadded directive

Author: javiereguiluz

components/string.rst

@@ -8,6 +8,11 @@ The String Component
     The String component provides a single object-oriented API to work with
     three "unit systems" of strings: bytes, code points and grapheme clusters.
 
+.. versionadded:: 5.0
+
+    The String component was introduced in 5.0 as an
+    :doc:`experimental feature </contributing/code/experimental>`.
+
 Installation
 ------------