Add paragraph on the two parts of mdast to `readme.md`
Add paragraph on the two parts of mdast to `readme.md`

tanzania82/remarks,eush77/remark,wooorm/remark,chcokr/mdast,yukkurisinai/mdast,eush77/remark,ulrikaugustsson/mdast,yukkurisinai/mdast,ulrikaugustsson/mdast,chcokr/mdast