Add introduction sub-section to 1.1 draft to enumerate new 1.1 features.

I think it would be helpful for those who are already familiar with 1.0 and want to see a abbreviated list of what significant features have been changed or added, all collected into one place. This new section would list the new features with a couple sentence high-level description of each. Each description would be followed by a reference to the section(s) of the document where they can go to find out more about that new feature. Perhaps this could be placed as a new part of our introduction section. A possible title would be, "Summary of significant changes since 1.0" followed by sub sections for Type Mutability, Repository Extensions, Secondary Types, etc.