Add underscore emphasis alternatives to Markdown guide

markdown.md

@@ -35,6 +35,17 @@ Use headings to structure documents, not for emphasis. Don't skip levels
 
 Renders as: **bold text**, *italic text*, ~~strikethrough~~, **bold and *italic* together**
 
+The same formatting can also be written with underscores:
+
+```markdown
+_italic_
+__bold__
+__bold and _italic_ together__
+```
+
+`*` and `_` are interchangeable. Convention is to use `*` for italic and `**`
+for bold — reserve `_` for nesting to avoid ambiguity.
+
 ## Lists
 
 ### Unordered