[gimp-web] content: add a warning about outdated book list and how to contribute.

[Jehan <jehanp src gnome org>]

commit ed1a1a99f3d32a8c06680eeb4bbafe29753ed96b
Author: Jehan <jehan girinstud io>
Date:   Wed Sep 29 12:36:26 2021 +0200

    content: add a warning about outdated book list and how to contribute.

 content/books/index.md | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/content/books/index.md b/content/books/index.md
index 83b21b78..a91c605d 100644
--- a/content/books/index.md
+++ b/content/books/index.md
@@ -3,6 +3,13 @@ Date: 2015-08-14T13:58:40-05:00
 Author: Pat David
 Status: hidden
 
+⚠️ *This page is quite outdated. If you wish to add a book about GIMP to
+this list, please [contribute modifications to this
+file](https://gitlab.gnome.org/Infrastructure/gimp-web/-/blob/testing/content/books/index.md).*
+
+*The file format is *markdown* (you may also look and copy the syntax
+used for listing other books). Keep the order by publication date.* ⚠️
+
 ## Beginning Photo Retouching & Restoration Using GIMP
 
 ![Beginning Photo Retouching & Restoration Using GIMP](apress-photo-retouching-restoration-gimp-2014.jpg)