Update sky/README.md to refer to Sky Widgets
Update sky/README.md to refer to Sky Widgets

Previously this README discussed fn.dart.

TBR=ianh@google.com

Review URL: https://codereview.chromium.org/1199753002.

chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,chinmaygarde/mojo,jianglu/mojo,jianglu/mojo,jianglu/mojo,jianglu/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,chinmaygarde/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,afandria/mojo,afandria/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,jianglu/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,afandria/mojo