Update README.md

Just wording - Slack is deprecated, Discord server is now official communication channel.

Added Hackster IO Links
Changed the link for the code.py to internal Donkey Car maintained version.

Co-authored-by: Tawn Kramer <tawnkramer@gmail.com>

joystic -> joystick

* consolidating config

* add default racer bio config to template

* multi_train changes

* Add rearlight support to mm1 circuitpython

* Add more validation on incoming calibration value

* Add stop sign detection

Co-authored-by: Jonathan Tse <jtse@proactivesystem.com.hk>

Add mobile app user guide

marge from dev branch

Make model type optional in makemovie and check model type if salient is true

2. If salient is true, check whether model type is linear or categorical. If not, exit and warn user.

Fix train script to correctly save best valuation loss model

Mm1 pwm value check

add missing dependency for code.py running on mm1 board

* Check point callback was initialised with on_best_model callback which overwrote the model on the end of each epoch, even when the valuation loss did not improve
* Fix passing on_best_model callback only if cfg.SEND_BEST_MODEL is set in the config
* Delete model.save() on_best_model as model saving is performed by Keras base class of MyCPCallback.