Add a 'Testing Features' section to the 'Download' page, describing h…

…ow to run the command and filter checking procedure.

html/download.html

@@ -484,5 +484,26 @@
 
     </div><div class="section_end"></div>
 
+    <!-- ******* -->
+    <!-- Testing Features -->
+    <!-- ******* -->
+    <a id="windows"></a>
+    <div class="section_title"><p>Testing Features</p></div><div class="section_content">
+      <p>In order to check if <span class="gmd_gmic">G'MIC</span> works correctly on your system,
+        you may want to execute the command and filter testing procedures. Assuming the CLI tool <tt>gmic</tt> is installed on your system,
+        here is how to do it (on an Unix-flavored OS, adapt the instructions below for other OS):
+      </p>
+      <div class="gmd_code_block">
+        $ mkdir -p testing && cd testing<br/>
+        $ gmic it https://gmic.eu/gmic_stdlib.\$_version parse_cli images<br/>
+        $ gmic it https://gmic.eu/gmic_stdlib.\$_version parse_cli images<br/>
+      </div>
+      <p>
+        These commands scan all G'MIC stdlib commands and G'MIC-Qt filters, and generate the images corresponding to the execution of these commands,
+        with default parameters. Beware, this may take some time to complete!
+      </p>
+
+    </div><div class="section_end"></div>
+
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