New section for annotated bibliography of motivations

[mhucka]

I previously shared this with people on Archivers' Slack on May 2, and previously had it located at https://mhucka.github.io/motivations-for-data-archiving/index.html

I finally figured out how to get collapsible topic headings in GitHub flavored Markdown, so now I can approximately the formatting I had before.

Moving here for easier editing and collaboration with everyone.