HACKING: Add Signed-off-by tag in the commit message

- Signed-off-by tag should be included in the commit message.
- Update the commit message example.

Signed-off-by: Kate Hsuan <hpa@redhat.com>

HACKING.md

@@ -79,10 +79,12 @@ cd contrib
   ```text
   === begin example commit ===
   short explanation of the commit  
-  
+
   Longer explanation explaining exactly what's changed, whether any
   external or private interfaces changed, what bugs were fixed (with bug
   tracker reference if applicable) and so forth. Be concise but not too brief.
+
+  Signed-off-by : Username <username@example.com>
   === end example commit ===
   ```
 
@@ -101,6 +103,8 @@ cd contrib
   git commit -a --author "Joe Coder <joe@coder.org>"
   ```
 
+- "Signed-off-by:" tags should be included" in the commit message.
+
 ## Coding Style
 
 - Please follow the coding style already used.