Add MD036 with tests.

doc/Rules.md

@@ -765,3 +765,32 @@ rules are different than the first one encountered in the document. If you
 want to configure the rule to match a specific style, the parameter given to
 the 'style' option is a string containing the exact horizontal rule text that
 is allowed.
+
+## MD036 - Emphasis used instead of a header
+
+Tags: headers, emphasis
+
+This check looks for instances where emphasized (i.e. bold or italic) text is
+used to separate sections, where a header should be used instead:
+
+    **My document**
+
+    Lorem ipsum dolor sit amet...
+
+    _Another section_
+
+    Consectetur adipiscing elit, sed do eiusmod.
+
+To fix this, use markdown headers instead of emphasized text to denote
+sections:
+
+    # My document
+
+    Lorem ipsum dolor sit amet...
+
+    ## Another section
+
+    Consectetur adipiscing elit, sed do eiusmod.
+
+Note: this rule looks for paragraphs that consist entirely of emphasized text.
+It won't fire on emphasis used within regular text.