text adapted from iPRES 2017 paper about Software Heritage, section "Data
Model"

Closes T787

Closes T1041

Closes: T335

Currently just a stub (but with a hereby fixed anchor) to allow linking to it
from other part of the development documentation. Will be fillen later on with
an abstract description of the Software Heritage data model.

change cherry picked from python module template
commit 71b117ba0cf9f1251b1cac26d0994df03a4c787d