Better docs for Vercel

## Expected

1. As a user I expect to be able to follow the guide so I can understand how to set up Ackee.
2. As a user I expect to be able to click the Vercel Button in order to deploy Ackee automatically

## Actual:

1. The docs don't explain what needs to be done to setup MongoDB
2. The Vercel Button clones the repo instead of forking it

It is unclear to me how to setup Ackee on Vercel.

I tried two options:

1. Vercel Button

This actually clones the repo instead of forking it.

2. Fork manually and deploy

This seems to be the better option since it allows to "Fetch upstream"

### Actual Problem with Docs:

What is mostly unclear and undocumented is: "Setting up MongoDB"

The docs mention:

> Set environment variables ACKEE_USERNAME, ACKEE_PASSWORD, ACKEE_MONGODB, and ACKEE_ALLOW_ORIGIN.

1. What should I put in `ACKEE_MONGODB`
2. What should I put in `ACKEE_ALLOW_ORIGIN`

Sorry if these questions seem dumb but I have no experience with MongoDB or Ackee.

It would be amazing if you could give me a hint on what needs to be done here.

I assume that `ACKEE_ALLOW_ORIGIN` would be the domain of the deployment but I'm unsure about it.
I have no idea what `ACKEE_MONGODB` does as it does not seem to be documented anywhere.
Do I need to setup a mongoDB instance myself or should this work out of the box?