Add note that to use KL divergence runwise bkgs need to be appended to all datastore runs

[samanthalwong]

Running KL_DIV = True gives an error (no "BACKGROUND" header attached to DL3 file) if the user has not generated runwise bkgs for datastore runs. Might be good to add somewhere that this needs to be done + point to script to do so to facilitate onboarding of new users (since it's a bit of a cryptic error).

[steob92]

This is already included in the docs:

https://github.com/VERITAS-Observatory/gammapy-tools/blob/main/docs/backgrounds.md

I guess we could do a check and return a link to the docs page?

[samanthalwong]

Oops I didn't see that! Will link in the notebook then