Update docs for setting up api keys locally
Add missing "v1" from url
Add existing post_id when slug already exists

- This will be useful for scripts, etc. that try to create a post and want to
  later send an update for a particular post.
- There's no way via the API to query via slug or even determine the slug that
  was automatically generated if one wasn't given.  So, this approach seems to
  be the smallest change with the most benefit.  Users of the API can easily
  parse this error message and then decide what to do with the existing ID.