doc/index.md: add an introductory paragraph
doc/index.md: add an introductory paragraph

Just steal text from README.md

Fixes #207.

Change-Id: Ie2a32ea8de3b5ccc9999ff30d48236b999f82eac
Reviewed-on: https://upspin-review.googlesource.com/7962
Reviewed-by: Bons <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@google.com>

upspin/upspin,upspin/upspin