Add `PlaceholderDesignable` in readme
Add `PlaceholderDesignable` in readme

alexiosdev/IBAnimatable,IBAnimatable/IBAnimatable,JakeLin/IBAnimatable,IBAnimatable/IBAnimatable,IBAnimatable/IBAnimatable,alexiosdev/IBAnimatable,seanwoodward/IBAnimatable,seanwoodward/IBAnimatable,JakeLin/IBAnimatable