document ability to use css selector
document ability to use css selector

closes #1168

willrowe/quill,sferoze/quilljs,sferoze/quilljs,willrowe/quill,quilljs/quill,sferoze/quilljs,voxmedia/quill,voxmedia/quill,sferoze/quilljs,quilljs/quill,quilljs/quill,voxmedia/quill,willrowe/quill