Docs: Requirements for Init script

`npx cssg init` already needs access to a Contentful space. In the README we could point out the possibilities (`contentful login` or `.env`).

If it is done manually via `.env`, then it is important to know that the management token is also needed.

Example:

```
CONTENTFUL_SPACE_ID = ''
CONTENTFUL_DELIVERY_TOKEN = ''
CONTENTFUL_PREVIEW_TOKEN = ''
CONTENTFUL_MANAGEMENT_TOKEN = ''
```