context: document configuration file format
context: document configuration file format

Change-Id: I891e7c524bbfdcb660392f9ed8f43e19bede26dd
Reviewed-on: https://upspin-review.git.corp.google.com/2004
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>

upspin/upspin,upspin/upspin