Document known issue

Closes sindresorhus#37

readme.md

@@ -789,6 +789,13 @@ Defaults[.stringSet].contains("Hello") //=> true
 Defaults[.stringSet].contains("World!") //=> true
 ```
 
+## Known issues
+
+### The preferences window doesn't show
+
+This can happen when you are not using auto-layout or have not set a size for the view controller. You can fix this by either using auto-layout or setting an explicit size, for example, `preferredContentSize` in `viewDidLoad()`. [We intend to fix this.](https://github.com/sindresorhus/Preferences/pull/28)
+
+
 ## FAQ
 
 ### How can I store a dictionary of arbitrary values?