Add Wiki documentation about gem deployment

[kinow]

Is your feature request related to a problem? Please describe.

Add docs about the deployment & testing process for the gem.

Describe the solution you'd like

Wiki page with info for maintainer/developers

Describe alternatives you've considered

Additional context

Notes

1. Check the Troubleshooting Guide in the wiki.
2. Ask in the Gitter Chat Room - please do not submit issues for support requests.
3. Use the GitHub Issue Search to check if the issue has already been reported.

[kinow]

WIP: https://github.com/Smashing/smashing/wiki/Gem-deployment

[kinow]

Done!