add Libraries section to README
add Libraries section to README

ktakashi/picrin,picrin-scheme/picrin,ktakashi/picrin,koba-e964/picrin,leavesbnw/picrin,picrin-scheme/picrin,koba-e964/picrin,dcurrie/picrin,koba-e964/picrin,dcurrie/picrin,leavesbnw/picrin,ktakashi/picrin,omasanori/picrin,omasanori/picrin,leavesbnw/picrin