Add note about Android left button icon constraint (#2963)

docs/adding-buttons-to-the-navigator.md

@@ -101,6 +101,7 @@ More information about these glyphs can be found in [Apple's Human Interface Gui
 
 
 ##### Android left button
+
 On Android, four button types are supported by default without the need to provide an icon. You can use them by specifying one of the following ids in your left button definition:
 
 * back
@@ -108,6 +109,8 @@ On Android, four button types are supported by default without the need to provi
 * accept
 * sideMenu
 
+Except for these 4 cases, the left button **must** have an icon on Android. It can't be a text only button.
+
 #### Custom Navigation Buttons
 
 On iOS, react-native-navigation uses `UIBarButtonItem` to display all items in the navigation bar. Instead of using images or text for normal `UIBarButtonItem`s, you can supply a custom component to be displayed within a custom view of a `UIBarButtonItem`, using the `component` property when specifying a navigation button.