Add docs for wrap-guide extension

docs/intro.md

@@ -1 +1,35 @@
 ## Welcome to the Atom guide
+
+## Extensions
+
+### Wrap Guide
+
+The wrap-guide extension places a vertical line in each editor at a certain
+column to guide your formatting so lines do not exceed a certain width.
+
+By default the wrap-guide is placed at the 80th column.
+
+#### Configuration
+
+You can configure where this column is on a per-path basis using the following
+configuration data options:
+
+```coffeescript
+wrapGuideConfig =
+  getGuideColumn: (path, defaultColumn) ->
+    if path.indexOf('.mm', path.length - 3) isnt -1
+      return -1 # Disable the guide for Objective-C files
+    else
+      return defaultColumn
+requireExtension 'wrap-guide', wrapGuideConfig
+```
+
+You can configure the color of the line by adding the following CSS to a custom
+stylesheet:
+
+```css
+.wrap-guide {
+  width: 10px;
+  background-color: red;
+}
+```