TF_VAR_SSH_PUBLIC_KEY not mentioned in the README.md

As it is well explained that the user has to define the TF_VAR_REGISTRATION_TOKEN variable, it should also be mentioned that the user has to define the TF_VAR_SSH_PUBLIC_KEY.

assigned to @tmartine

mentioned in commit 498eb8d9

mentioned in merge request !2 (merged)

Thank you for noticing that, @arrivaul! Fixed here: !2 (merged)

mentioned in commit fd92b349

closed with commit 498eb8d9