add react-testing-library documentation/examples (#4679)
add react-testing-library documentation/examples (#4679)

* add react-testing-library documentation/examples

* make react-testing-library a heading

* fix typo

timlogemann/create-react-app,Timer/create-react-app,romaindso/create-react-app,liamhu/create-react-app,viankakrisna/create-react-app,viankakrisna/create-react-app,facebookincubator/create-react-app,d3ce1t/create-react-app,amido/create-react-app,romaindso/create-react-app,GreenGremlin/create-react-app,IamJoseph/create-react-app,GreenGremlin/create-react-app,d3ce1t/create-react-app,jdcrensh/create-react-app,facebookincubator/create-react-app,infernojs/create-inferno-app,ConnectedHomes/create-react-web-app,Timer/create-react-app,jdcrensh/create-react-app,IamJoseph/create-react-app,mangomint/create-react-app,infernojs/create-inferno-app,liamhu/create-react-app,amido/create-react-app,jdcrensh/create-react-app,timlogemann/create-react-app,timlogemann/create-react-app,mangomint/create-react-app,timlogemann/create-react-app,d3ce1t/create-react-app,Timer/create-react-app,amido/create-react-app,mangomint/create-react-app,Timer/create-react-app,romaindso/create-react-app,GreenGremlin/create-react-app,GreenGremlin/create-react-app,mangomint/create-react-app,d3ce1t/create-react-app,ConnectedHomes/create-react-web-app,infernojs/create-inferno-app,ConnectedHomes/create-react-web-app,jdcrensh/create-react-app,liamhu/create-react-app,facebookincubator/create-react-app,viankakrisna/create-react-app,ConnectedHomes/create-react-web-app,IamJoseph/create-react-app