Document the transparent option

sindresorhus · sindresorhus · commit f96c07e457cc · 2017-06-15T11:33:00.000+02:00
diff --git a/readme.md b/readme.md
@@ -164,6 +164,13 @@ Type: `string`
 
 Custom user agent.
 
+### transparent
+
+Type: `Boolean`<br>
+Default: `false`
+
+Set background color to `transparent` instead of `white` if no background is set.
+
 ### headers
 
 Type: `object`
