Coding Style Guide: add 'Nullability Annotations' section
Coding Style Guide: add 'Nullability Annotations' section

Also:
- Make new separate section for Header File Example.
- Other very small fixes.

CaueAlvesSilva/ResearchKit,boland25/ResearchKit,jeremiahyan/ResearchKit,allen7wang/ResearchKit,chb/ResearchKit,oliverschaefer/ResearchKit,cornellradiology/ResearchKit,qualwas72/ResearchKit,jarrelscy/ResearchKit,danoli3/ResearchKit,YuanZhu-apple/ResearchKit,CaueAlvesSilva/ResearchKit,cornelltech/ResearchKit,QuintilesRK/ResearchKit,jarrelscy/ResearchKit,zdw19840929/ResearchKit,CaueAlvesSilva/ResearchKit,chb/ResearchKit,cnbin/ResearchKit,oliverschaefer/ResearchKit,jimmyti/ResearchKit,qualwas72/ResearchKit,jimmyti/ResearchKit,QuintilesRK/ResearchKit,oliverschaefer/ResearchKit,jeremiahyan/ResearchKit,weijentu/ResearchKit,shazino/ResearchKit,zhuzhengwei/ResearchKit,KaiCode2/ResearchKit,YuanZhu-apple/ResearchKit,QuintilesRK/ResearchKit,umerkhan-apple/ResearchKit,allen7wang/ResearchKit,DrGo/ResearchKit,zhuzhengwei/ResearchKit,qualwas72/ResearchKit,oliverschaefer/ResearchKit,md0u80c9/ResearchKit,lijie121210/ResearchKit,shazino/ResearchKit,jimmyti/ResearchKit,danoli3/ResearchKit,boland25/ResearchKit,danoli3/ResearchKit,zdw19840929/ResearchKit,cnbin/ResearchKit,YuanZhu-apple/ResearchKit,boland25/ResearchKit,md0u80c9/ResearchKit,zhuzhengwei/ResearchKit,umerkhan-apple/ResearchKit,DrGo/ResearchKit,shazino/ResearchKit,allen7wang/ResearchKit,cornelltech/ResearchKit,zdw19840929/ResearchKit,boland25/ResearchKit,mluedke2/ResearchKit,mluedke2/ResearchKit,lijie121210/ResearchKit,cornellradiology/ResearchKit,lijie121210/ResearchKit,zdw19840929/ResearchKit,wlads/ResearchKit,KaiCode2/ResearchKit,cornellradiology/ResearchKit,CaueAlvesSilva/ResearchKit,mluedke2/ResearchKit,chb/ResearchKit,md0u80c9/ResearchKit,chb/ResearchKit,qualwas72/ResearchKit,zhuzhengwei/ResearchKit,weijentu/ResearchKit,jeremiahyan/ResearchKit,umerkhan-apple/ResearchKit,weijentu/ResearchKit,KaiCode2/ResearchKit,cornellradiology/ResearchKit,jarrelscy/ResearchKit,wlads/ResearchKit,cornelltech/ResearchKit,weijentu/ResearchKit,YuanZhu-apple/ResearchKit,md0u80c9/ResearchKit,wlads/ResearchKit,DrGo/ResearchKit,lijie121210/ResearchKit,cnbin/ResearchKit