add xmake.io to README.md
add xmake.io to README.md

waruqi/xmake,waruqi/xmake,tboox/xmake,waruqi/xmake,tboox/xmake,waruqi/xmake,tboox/xmake,waruqi/xmake,tboox/xmake,waruqi/xmake,tboox/xmake,tboox/xmake,waruqi/xmake,tboox/xmake