reftable: document reading and writing indices

reftable: document reading and writing indices

The way the index gets written and read is not trivial at all and
requires the reader to piece together a bunch of parts to figure out how
it works. Add some documentation to hopefully make this easier to
understand for the next reader.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>