Add initial nullability annotations guidance (#36711)
Add initial nullability annotations guidance (#36711)

* Add initial nullability annotations guidance

* Address PR feedback

ericstj/corefx,shimingsg/corefx,BrennanConroy/corefx,shimingsg/corefx,shimingsg/corefx,ViktorHofer/corefx,ericstj/corefx,wtgodbe/corefx,ViktorHofer/corefx,ViktorHofer/corefx,wtgodbe/corefx,shimingsg/corefx,ViktorHofer/corefx,shimingsg/corefx,ericstj/corefx,shimingsg/corefx,BrennanConroy/corefx,wtgodbe/corefx,ViktorHofer/corefx,ericstj/corefx,ericstj/corefx,BrennanConroy/corefx,ViktorHofer/corefx,wtgodbe/corefx,ViktorHofer/corefx,wtgodbe/corefx,ericstj/corefx,wtgodbe/corefx,shimingsg/corefx,wtgodbe/corefx,ericstj/corefx