P276 provides a configuration file for the following config classes content:

• Nasqueron\SAAS\MediaWiki\Configuration\Instances::getList()
• Nasqueron\SAAS\MediaWiki\Configuration\Instances::getAliases()
• Nasqueron\SAAS\MediaWiki\Configuration\Settings::getDatabaseMap()
• Nasqueron\SAAS\MediaWiki\Configuration\Settings::getMappedSettings()
• Nasqueron\SAAS\MediaWiki\WithLog::getLoggerConfiguration()

The content is up to date with commit 6cc6f31e284b (current master)

Yaml is a good format, as it's compact, readable, doesn't require to play with comma, quotes, brackets, etc. and allows documentation (JSON doesn't).