[gspell] docs: readme: gspell is for GTK 3

[Sébastien Wilmet <swilmet src gnome org>]

commit 6c1c9d8b7eb4441070cd16c0672000b4dbe493e3
Author: Sébastien Wilmet <swilmet informatique-libre be>
Date:   Fri Apr 15 18:16:41 2022 +0200

    docs: readme: gspell is for GTK 3

 README.md | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/README.md b/README.md
index d15d755..b60bb8e 100644
--- a/README.md
+++ b/README.md
@@ -18,4 +18,6 @@ Features
 More information
 ----------------
 
+The gspell 1.x versions target **GTK 3**.
+
 [More information about gspell](docs/more-information.md).