Document new build options

[maxgerhardt]

The USB libraries are e.g. looking for some macros which will activate the respective USB functionality / core / class

gd32-pio-spl-package/gd32/spl/libraries/gd32f3x0/GD32F3x0_usbfs_library/build_usb.py

Lines 5 to 15 in 93fcfd7

	device_flags_to_sources = {
	"PIO_USBFS_DEVICE_AUDIO": "audio",
	"PIO_USBFS_DEVICE_CDC": "cdc",
	"PIO_USBFS_DEVICE_DFU": "dfu",
	"PIO_USBFS_DEVICE_HID_STANDARD": "hid",
	"PIO_USBFS_DEVICE_HID_CUSTOM": "hid",
	"PIO_USBFS_DEVICE_IAP": "iap",
	"PIO_USBFS_DEVICE_MSC": "msc",
	"PIO_USBFS_DEVICE_PRINTER": "printer"
	}

gd32-pio-spl-package/gd32/spl/libraries/gd32f3x0/GD32F3x0_usbfs_library/build_usb.py

Lines 31 to 34 in 93fcfd7

	host_flags_to_sources = {
	"PIO_USBFS_HOST_HID": "hid",
	"PIO_USBFS_HOST_MSC": "msc"
	}

This needs to be documented.

Also good would be general documentation about the libraries and how to add them into the project / work with them.