docs: add a basic README file

README.md

+swh-model
+=========
+
+Implementation of the Data model of the Software Heritage project, used to
+archive source code artifacts.
+
+This module defines the notion of Persistent Identifier (PID) and provides
+tools to compute them:
+
+```sh
+   $ swh-identify fork.c kmod.c sched/deadline.c
+   swh:1:cnt:2e391c754ae730bd2d8520c2ab497c403220c6e3    fork.c
+   swh:1:cnt:0277d1216f80ae1adeed84a686ed34c9b2931fc2    kmod.c
+   swh:1:cnt:57b939c81bce5d06fa587df8915f05affbe22b82    sched/deadline.c
+
+   $ swh-identify --no-filename /usr/src/linux/kernel/
+   swh:1:dir:f9f858a48d663b3809c9e2f336412717496202ab
+```