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