BFM demod: added help page to GUI

plugins/channelrx/demodbfm/bfmdemodgui.cpp

@@ -365,6 +365,7 @@ BFMDemodGUI::BFMDemodGUI(PluginAPI* pluginAPI, DeviceUISet *deviceUISet, Baseban
 	m_rate(625000)
 {
 	ui->setupUi(this);
+	m_helpURL = "plugins/channelrx/demodbfm/readme.md";
     ui->deltaFrequencyLabel->setText(QString("%1f").arg(QChar(0x94, 0x03)));
     ui->deltaFrequency->setColorMapper(ColorMapper(ColorMapper::GrayGold));
     ui->deltaFrequency->setValueRange(false, 8, -99999999, 99999999);