[erl-frontend] Extend test readme a bit
[erl-frontend] Extend test readme a bit

Summary: Add some more details to readme about Erlang tests.

Reviewed By: kren1

Differential Revision: D38569355

fbshipit-source-id: fa8aa017c650d08f30acb23ba79143108a434012

facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer