Further refine README docs
Further refine README docs

- Improve intro
- Move responsive images support section
- Improve section related to auto-sizing

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