README.md: add 'Charts' section
README.md: add 'Charts' section

Also add information about the chart examples in ORKCatalog.

YuanZhu-apple/ResearchKit,weijentu/ResearchKit,allen7wang/ResearchKit,md0u80c9/ResearchKit,YuanZhu-apple/ResearchKit,md0u80c9/ResearchKit,oliverschaefer/ResearchKit,chb/ResearchKit,cornelltech/ResearchKit,YuanZhu-apple/ResearchKit,cornelltech/ResearchKit,KaiCode2/ResearchKit,oliverschaefer/ResearchKit,boland25/ResearchKit,weijentu/ResearchKit,chb/ResearchKit,cornelltech/ResearchKit,md0u80c9/ResearchKit,oliverschaefer/ResearchKit,YuanZhu-apple/ResearchKit,allen7wang/ResearchKit,weijentu/ResearchKit,allen7wang/ResearchKit,mluedke2/ResearchKit,oliverschaefer/ResearchKit,weijentu/ResearchKit,boland25/ResearchKit,KaiCode2/ResearchKit,boland25/ResearchKit,chb/ResearchKit,jeremiahyan/ResearchKit,mluedke2/ResearchKit,KaiCode2/ResearchKit,boland25/ResearchKit,mluedke2/ResearchKit,jeremiahyan/ResearchKit,md0u80c9/ResearchKit,jeremiahyan/ResearchKit,chb/ResearchKit