Add 'when to use' section and reorder sections
Add 'when to use' section and reorder sections

ermakovmaxim/lazysizes,haidaxiaocong/lazyload,phpgit/lazysizes,cnweibo/lazysizes,phpgit/lazysizes,haidaxiaocong/lazyload,aFarkas/lazysizes,ristoman/lazysizes,ristoman/lazysizes,billybonz1/lazysizes,billybonz1/lazysizes,youprofit/lazysizes,aFarkas/lazysizes,ermakovmaxim/lazysizes,youprofit/lazysizes,cnweibo/lazysizes