Add upgrading note to README
Add upgrading note to README

hudochenkov/stylelint,stylelint/stylelint,heatwaveo8/stylelint,hudochenkov/stylelint,gucong3000/stylelint,stylelint/stylelint,gaidarenko/stylelint,gaidarenko/stylelint,gucong3000/stylelint,m-allanson/stylelint,stylelint/stylelint,gucong3000/stylelint,heatwaveo8/stylelint,stylelint/stylelint,hudochenkov/stylelint,heatwaveo8/stylelint,evilebottnawi/stylelint,evilebottnawi/stylelint,gaidarenko/stylelint