Added description and code example for video attachments.

doc.zih.tu-dresden.de/docs/contrib/content_rules.md

@@ -129,6 +129,8 @@ Further tips can be found on this
 Please use images and graphics for illustration purposes and to improve comprehensibility.
 All graphics and attachments are saved within `misc` directory of the respective subdirectory in
 `docs`.
+For video attachments please use either webm or mp4 format, in fact it would be best to upload one
+file each in webm and mp4 because then you cover a wider range of browsers.
 
 !!! example "Syntax"
 
@@ -139,6 +141,17 @@ All graphics and attachments are saved within `misc` directory of the respective
     {: align="center"}
     ```
 
+    The syntax to insert a video attachment into a page is
+
+    ```html
+    <video controls="" width="320" style="border: 1px solid black">
+        <source src="https://doc.zih.tu-dresden.de/software/misc/terminate-virtual-desktop-dcv.mp4"
+            type="video/mp4" />
+        <source src="https://doc.zih.tu-dresden.de/software/misc/terminate-virtual-desktop-dcv.webm"
+            type="video/webm" />
+    </video>
+    ```
+
 The attribute `align` is optional. By default, graphics are left-aligned. **Note:** It is crucial to
 have `{: align="center"}` on a new line.
 It is possible to add captions for tables and figures using `{: summary="This is a table caption"}`.