[Wiki] Add more tutorials and use cases

[duonglx]

Hi PM,

Your project is really great. Can you add more tutorials and use cases so that people can understand and refer to the application of Gateway?
I think that would be very helpful and great with us.

Thank you so much!

[grozadanut]

Subscribe to what duonglx said, it would be nice to have the needed SMTP configuration in the wiki as well. Although I could probably find the needed settings by looking at the code, albeit a bit slower. Also, is there a space(chat, forum, mailing list) for developers to discuss with each other?