Move discussion channel from slack to github discussion

[MainRo]

This is a proposal: The current official discussion channel for RxPY is on slack. However posts are rare there, and answers are often quite long. On the other way, many people use the Github issues to ask for questions (much more than on slack!).
It may be more natural to use GitHub discussions for this.

[MainRo]

Thanks @dbrattli !
I can update the readme accordingly to favor GitHub discussions over the slack channel.

[dbrattli]

@MainRo I already did some changes to the readme. Please feel free to add more info.