Document how to patch the module from pull requests

[mglaman]

Package

next (Drupal module)

Describe the feature request

This is a mono-repo, so patching the Drupal module can be difficult as the structure doesn't match the split to Drupal.org. Provide information on how to patch from the GitHub repo.

Describe the solution you'd like

Document that patchLevel of -p3 needs to be applied for cweagans/composer-patches

        "patchLevel": {
            "drupal/core": "-p2",
            "drupal/next": "-p3"
        },

Additional context

We need #606 to prevent crashes and had to apply it somehow.

[mglaman]

I actually have no idea where this would be go. Maybe the README? Maybe documentation? Maybe this just gets moved to a Discussion as a form of record, there.

[JohnAlbin]

There's a CONTRIBUTING.md doc now.