Flesh out the docs on adonis-ally

[johnpyp]

I recently started with adonis, and the first thing I wanted to do was get some kind of social authentication going, and there were a few pain points that took me a while to solve and I think would be valuable additions to the social auth docs.

1. I think it would be useful to go over specifically what you need to setup your database to handle user authentication. A basic review covering just the barebones users migration and user model (noting it can be empty and still work). I feel like the docs are lacking in this regard, repetition isn't used as much as it could to improve understanding.

2. Add in the common use case of an already-logged-in guard to the "basic example", something like this:

      try {
        await auth.check()
      } catch (error) {
        await auth.login(user)
      }

3. Add a "show/getUser" method to the example, showing how to retrieve the current user once you are authenticated, like this:

  async show({ auth, params }) {
    try {
      let user = await auth.getUser()
      return { status: true, data: user }
    } catch (e) {
      return { status: false, data: 'Not Logged In' }
    }
  }

4. Add a note about the session config option sameSite. I don't know if this is a consistent issue, but using google auth I had a huge issues and I had no idea about this until I found an obscure issue regarding it.

5. Add an example about the remember(true) setting, because it seems to need a token model and migration setup with the user. I still haven't figured out how to set this up, though I didn't need it that badly.