Feat: Add slack etiquettes to the contributing.md file

[Honyii]

As per this issue raised by @mamoodi here, we will need to add slack etiquettes to the contributing.md file. Also, i would be working to create this pr

[mamoodi]

I don't think contributing.md is the right place for it but that's details. You can put up a PR for review and then I can ping people to take a look.