Document using database-driven backends

**- Do you want to request a *feature* or report a *bug*?**
*Feature*

**- What is the current behavior?**
Only Git based backend repo like GitHub, GitLab, or Bitbucket is supported

**- If the current behavior is a bug, please provide the steps to reproduce.**

**- What is the expected behavior?**
It will be fantastic if there are some pointers to create a custom backend with S3 compatible storage like [Minio](https://minio.io)

* There seems to be a prior discussion in decaporg/decap-cms#616
* Another related discussion on a PR decaporg/decap-cms#786
* But with v2.0, will the customization be isolated to a particular package? Any pointers or tips would be really useful. Thank you !