doc: update wording on front page
doc: update wording on front page

Say some more meaningful stuff on the front page.

Change-Id: I408b176793c5087b0caddeeec48786b3a075f1ee
Reviewed-on: https://upspin-review.googlesource.com/11642
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin