CONFIGURATION.md: Add nodeless and opennode to payments.processor docs #517

The CONFIGURATION.md file's settings table for payments.processor currently lists only zebedee, lnbits, and lnurl as valid values:

payments.processor | Either zebedee, lnbits, lnurl.

But the code supports nodeless and opennode as well, and their configuration sections are present in the same documentation. Please update the documentation to include all supported values (zebedee, lnbits, lnurl, nodeless, opennode).

This ensures the settings documentation matches actual code capabilities and avoids user confusion.