[Keyboard] niu_mini uses dfu bootloader rather than the afrdude bootloader (#6092)
* niu_mini uses dfu bootloader rather than the afrdude bootloader
modified: readme.md
* Change rules in rules.mk to reflect the bootloader change
modified: keyboards/niu_mini/rules.mk
Make example for this keyboard (after setting up your build environment):
make niu_mini:default:avrdude
make niu_mini:default:dfu
See [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) then the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information.