fix material icons

README.md

  you can refer to Flutter Wiki to access detailed descriptions of UIWidgets APIs 
  from those of their Flutter counterparts.
  
+
+#### NOTES
+##### Using Material Icons
+To use material Icons, you need to add font MaterialIcons-Regular.ttf into your project and add it into 
+```FontManager``` as the sample code shows. The file name must be MaterialIcons-Regular.ttf, otherwise the icons 
+could not shown 
+
 #### FAQ
 
 | Question     | Answer  |