Updating numbered list guidelines

To avoid https://github.com/asciidoctor/asciidoctor/issues/3252
diff --git a/netbeans.apache.org/src/content/kb/docs/contributing.asciidoc b/netbeans.apache.org/src/content/kb/docs/contributing.asciidoc
index 0bebf8e..7f489b2 100644
--- a/netbeans.apache.org/src/content/kb/docs/contributing.asciidoc
+++ b/netbeans.apache.org/src/content/kb/docs/contributing.asciidoc
@@ -239,10 +239,12 @@
 
 Now we have a paragraph here, or an image or whatever
 
-[start=3]
-3. This is the third item
+[start=3] 
+. This is the third item
 ----
 
+<1>: Note that since we have a `[start=3]` preamble, we don't add a number before the dot, this is, we write `. This is the third item` and not `3. This is the third item`. By doing so we avoid this link:https://github.com/asciidoctor/asciidoctor/issues/3252[asciidoctor warning].
+
 === Admonitions (notes, tips)
 
 Use `NOTE:` and `TIP:` for admonitions. These must be in capital letters, without any asterkisks around them: