Add in the guide how to set up the flatnode file

[gsg-git]

I think there where more than just me wo had problems with too big databases.
So it would be nice to kow how to set the flatnode file at the beginning at creating the container command.
And it should be made more clear how important this file is.

For me it was like an optional feature like an cache function in the beginnig.
It would be nice if it would be more obviously for everyone else in the future.

Thank you!

[gsg-git]

Im working on a PR for the redame