update readme
update readme

* add detailed API section
* remove the list of clean-css options
* add a link to original API doc
* remove Caveats section since jakubpawlowicz/clean-css#397 is fixed

sibrus/gulp-minify-css,zarko-tg/gulp-minify-css,keremciu/gulp-minify-css,LoicMahieu/gulp-minify-css,HaNdTriX/gulp-minify-css,MichaelFBA/gulp-minify-css