Add postcss-assets-rebase plugin to readme
Add postcss-assets-rebase plugin to readme

jonathantneal/postcss,postcss/postcss,andrepolischuk/postcss,XOP/postcss,jedmao/postcss,bezoerb/postcss,Semigradsky/postcss,postcss/postcss,maximkoretskiy/postcss,pascalduez/postcss,dehuszar/postcss,alanev/postcss