Create guide to create a GitHub App that uses Reposaur

[wtfiscrq]

Apart from using Reposaur in the CLI (and for instance in Github Actions), it can also be used as a GitHub App. Using Reposaur as a GitHub App means we don't have to add a workflow to every repository nor create an org-wide repository to run the policies.

Reposaur exports a SDK that makes it possible and a guide on how to create one would be nice not only for the sake of it but also for guiding people on how to use the SDK to use/extend Reposaur base functionality.

A GitHub App as an AWS Lambda would be a great fit.