Add a warning about stamps into the docs

doc/index.docbook

@@ -34,8 +34,8 @@ Context menu actions like Rename Bookmarks etc.)
 			<!-- TRANS:ROLES_OF_TRANSLATORS -->
 		</authorgroup>
 		<legalnotice>&FDLNotice;</legalnotice>
-		<date>2019-06-24</date>
+		<date>2019-07-26</date>
-		<releaseinfo>1.8 (Applications 19.08)</releaseinfo>
+		<releaseinfo>1.9 (Applications 19.12)</releaseinfo>
 		<!-- Abstract about this handbook -->
 		<abstract>
 			<para>&okular; is a &kde; universal document viewer based on &kpdf; code.</para>
@@ -730,8 +730,13 @@ Context menu actions like Rename Bookmarks etc.)
 					    To add a rectangular stamp you can click with the &LMB; and hold to place the top-left point, then drag to place the bottom-right one.
 					  </para>
 					  <para>
-                          It is possible to define the <guilabel>Opacity</guilabel> and <guilabel>Stamp symbol</guilabel> for the stamp. Just right-click on the stamp icon created and select the <guimenuitem>Properties</guimenuitem> menu item.
+					    It is possible to define the <guilabel>Opacity</guilabel> and <guilabel>Stamp symbol</guilabel> for the stamp. Just right-click on the stamp icon created and select the <guimenuitem>Properties</guimenuitem> menu item.
 					  </para>
+					  <warning>
+					  	 <para>
+					  	 	 This feature is experimental. Custom stamps inserted in &PDF; documents are not visible in &PDF; readers other than &okular;.
+					  	 </para>
+					  </warning>
 					</entry>
 				      </row>
 				      <row>