README: made for Teensy 3.x and 4.x

README: recommend Teensy 4.1, then Teensy 3.6

README: no longer recommend the Teensy++ 2.0

README: Teensy++ 2.0 is now discontinued

README: mention QMK 0.14.0 release which includes kint36 and kint4*

README: point people to `develop` QMK branch This is hopefully less confusing than specifying the version without a branch: https://github.com/kinx-project/kint/commit/6accacf19ab8aaf72e1c498f3b11d43e0be25921

recommend QMK 0.13.17 or newer Version 0.13.17 contains bug fixes for kint2pp and kint36: https://github.com/qmk/qmk_firmware/pull/13351/commits/8ddb7f898ae6af9d067bdddea4675b76aec24e9b (kint41 already has the fix)

README: kint41 (Teensy 4.0 and 4.1) now supported by QMK! 🎉

README: link to QMK Configurator

README: clarify LED debugging step only works without Teensy