[spec] add README for the inputs and outputs

spec/input/README.md

+How to update input files:
+==========================
+
+data_* files:
+-------------
+Uncomment the corresponding `gen_stub()` line in ../oar_properties2_spec.rb and re-run rspec.
+
+oar_* files:
+------------
+Except the oar_empty.json file which should not have to be regenerated, the other files must be forged from the output of the OAR API.
+
+Steps are:
+- fetch the OAR API output for the site: `curl 'https://api.grid5000.fr/stable/sites/nancy/internal/oarapi/resources/details.json?limit=999999' > oar_site.json`
+- sort the objects with jq: `cat oar_site.json | jq -S "." > new_oar_site.json; mv {new_,}oar_site.json`
+- replace the tested cluster names, e.g. with sed: `sed -ie 's/graffiti/clustera/g' oar_site.json`.
+- modify the oar_site.json file as needed for what should be tested.
+
+Since using `vimdiff` or `meld` on the files is very slow, one can use the `diff` and `patch` commands...
+
+Then verify using `git diff` that the generated data matches what you expect.

spec/output/README.md

+How to update the output files:
+===============================
+
+To update the files, just remove them and run rspec again, they will be regenerated.
+Then verify using `git diff` that the generated data matches what you expect.