Document how to create a custom package

To test some changes in the incoming email feature we need to include a specific commit of a gem that is just not released, and may not even be released ever.

@jacobvosmaer suggested to create a custom package so we can deploy it instead of fiddling with production to get it to the version that was required.

So the outcome of this task is that we have documentation on how to build and deploy a custom package that only changes the gem that we need to change.

cc/ @jnijhof