Update test-markdown.md

doc/developer/tests/test-markdown.md

@@ -5,6 +5,10 @@ Paste the following tests into Mattermost to test markdown support.
 ```
 # This should render as Heading 1 font size 
 ## This should render as Heading 2 font size 
+### This should render as Heading 3 font size 
+#### This should render as Heading 4 font size 
+##### This should render as Heading 5 font size 
+###### This should render as Heading 6 font size 
 ~~This should show strikethrough formatting~~
 **This should be bold**
 ```