Add test directions

[muellerzr]

Adds a small markdown file that contains directions for running the test suite, since it took me a moment to get everything setup on my side.

[osanseviero]

If you do export HUGGINGFACE_CO_STAGING=1 that will impact you later unless you revert it.

One solution is to do
HUGGINGFACE_CO_STAGING pytest -sv ./tests/
and then it should work.

[osanseviero]

I'm ok with going with this proposal in /tests/README/md for now, but we should consider creating a CONTRIBUTING.md for this.

[muellerzr]

Should be good to go now @osanseviero :)

[osanseviero]

Thanks for this!