Create test-markdown.md

doc/developer/tests/test-markdown.md

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+# Markdown tests
+
+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 show strikethrough formatting~~
+**This should be bold**
+```