Update README so it's not mobile-specific (#26057)
Update README so it's not mobile-specific (#26057)

Flutter now covers many non-mobile platforms such as web, desktop, and
embedded scenarios. This updates the introductory text in the README to
match the wording used in the framework README.
jamesr/sky_engine,jamesr/sky_engine,chinmaygarde/flutter_engine,Hixie/sky_engine,jason-simmons/flutter_engine,jamesr/flutter_engine,flutter/engine,flutter/engine,devoncarew/engine,rmacnak-google/engine,Hixie/sky_engine,jamesr/flutter_engine,rmacnak-google/engine,devoncarew/engine,jason-simmons/flutter_engine,jamesr/flutter_engine,devoncarew/sky_engine,devoncarew/sky_engine,chinmaygarde/flutter_engine,devoncarew/engine,Hixie/sky_engine,chinmaygarde/flutter_engine,aam/engine,jamesr/flutter_engine,devoncarew/sky_engine,devoncarew/engine,jamesr/flutter_engine,flutter/engine,rmacnak-google/engine,jamesr/flutter_engine,chinmaygarde/flutter_engine,rmacnak-google/engine,Hixie/sky_engine,devoncarew/engine,devoncarew/engine,aam/engine,jamesr/sky_engine,jamesr/sky_engine,flutter/engine,jamesr/sky_engine,jamesr/flutter_engine,jamesr/flutter_engine,flutter/engine,jason-simmons/flutter_engine,aam/engine,flutter/engine,jason-simmons/flutter_engine,devoncarew/sky_engine,flutter/engine,rmacnak-google/engine,jason-simmons/flutter_engine,aam/engine,Hixie/sky_engine,jamesr/flutter_engine,aam/engine,devoncarew/sky_engine,aam/engine,devoncarew/engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,Hixie/sky_engine,aam/engine,flutter/engine,chinmaygarde/flutter_engine,jamesr/sky_engine,rmacnak-google/engine,Hixie/sky_engine,Hixie/sky_engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,devoncarew/sky_engine,aam/engine,devoncarew/sky_engine,rmacnak-google/engine,jamesr/sky_engine