Add documentation for configuration file format

[synthead]

nload supports a configuration file, but the format and options are not documented.

The keys can be found here:

nload/src/app.cpp

Lines 58 to 100 in 8f92dc0

	// create setting infrastructure
	SettingStore::add(Setting("AverageWindow", "Window length for average (s)", STANDARD_AVERAGE_WINDOW));
	SettingStore::add(Setting("BarMaxIn", "Max Incoming deflection (kBit/s)", STANDARD_MAX_DEFLECTION));
	SettingStore::add(Setting("BarMaxOut", "Max Outgoing deflection (kBit/s)", STANDARD_MAX_DEFLECTION));
	SettingStore::add(Setting("DataFormat", "Unit for data numbers", STANDARD_DATA_FORMAT));
	SettingStore::add(Setting("Devices", "Devices to show", "all"));
	SettingStore::add(Setting("MultipleDevices", "Show multiple devices", STANDARD_HIDE_GRAPHS));
	SettingStore::add(Setting("RefreshInterval", "Refresh interval (ms)", STANDARD_REFRESH_INTERVAL));
	SettingStore::add(Setting("TrafficFormat", "Unit for traffic numbers", STANDARD_TRAFFIC_FORMAT));
	SettingStore::get("AverageWindow").pushFilter(new SettingFilterMin(5));
	SettingStore::get("BarMaxIn").pushFilter(new SettingFilterAllowValue("0"));
	SettingStore::get("BarMaxIn").pushFilter(new SettingFilterMin(10));
	SettingStore::get("BarMaxOut").pushFilter(new SettingFilterAllowValue("0"));
	SettingStore::get("BarMaxOut").pushFilter(new SettingFilterMin(10));
	SettingStore::get("RefreshInterval").pushFilter(new SettingFilterMin(50));
	SettingStore::get("Devices").pushFilter(new SettingFilterDefault("all"));
	SettingStore::get("Devices").pushFilter(new SettingFilterExclusive("all"));
	map<string, string> valueMapping;
	valueMapping[toString(false)] = "[ ]";
	valueMapping[toString(true)] = "[x]";
	SettingStore::get("MultipleDevices").pushFilter(new SettingFilterMap(valueMapping));
	valueMapping.clear();
	valueMapping[toString(Statistics::humanReadableBit)] = "Human Readable (Bit)";
	valueMapping[toString(Statistics::humanReadableByte)] = "Human Readable (Byte)";
	valueMapping[toString(Statistics::bit)] = "Bit";
	valueMapping[toString(Statistics::byte)] = "Byte";
	valueMapping[toString(Statistics::kiloBit)] = "kBit";
	valueMapping[toString(Statistics::kiloByte)] = "kByte";
	valueMapping[toString(Statistics::megaBit)] = "MBit";
	valueMapping[toString(Statistics::megaByte)] = "MByte";
	valueMapping[toString(Statistics::gigaBit)] = "GBit";
	valueMapping[toString(Statistics::gigaByte)] = "GByte";
	SettingStore::get("TrafficFormat").pushFilter(new SettingFilterMap(valueMapping));
	SettingStore::get("DataFormat").pushFilter(new SettingFilterMap(valueMapping));
	valueMapping.clear();
	// load system and user settings
	loadSettings();

...and the configuration can be set using an ini format of key = value, but I was only able to learn this from reading the source code.