Update README.md
Improved parent/child control relationships.
Fix placeholder, add controls, static/bound prop.

PlaceHolder was making bad assumptions on where to move controls from on
AddControlTo and RemoveControlFrom.

Added CheckBox and GridButtonColumn.

Added StaticBinding so that certain bound properties may also be
statically set (such as the Text property on the Label control).

Added delete button to GridPage example.
Add grid sorting.