Add note about postcss 5/4 compatibility to readme
Add note about postcss 5/4 compatibility to readme

stylelint/stylelint,heatwaveo8/stylelint,hudochenkov/stylelint,MadLittleMods/stylelint,evilebottnawi/stylelint,m-allanson/stylelint,gucong3000/stylelint,mahtd/stylelint,heatwaveo8/stylelint,stylelint/stylelint,gaidarenko/stylelint,hudochenkov/stylelint,gaidarenko/stylelint,hudochenkov/stylelint,teryaew/stylelint,stylelint/stylelint,stylelint/stylelint,gucong3000/stylelint,heatwaveo8/stylelint,evilebottnawi/stylelint,gucong3000/stylelint,gaidarenko/stylelint