Include a script to allow developers submitting apps to pre-validate their PR

[tangollama]

Summary

From @Kav91:
Could we have a “catalog-ready” script added, like a cut down version off validate-catalog-requirements? So say before someone tries to add the submodule, in the steps you tell them to run the script to ensure it has everything in the checklist. Then anything missed can be linked back to the docs as the message.

I think it could save some time in the PR process.