Expand Circle-CI Tutorial & add to Tech Stack

[nelsonic]

Sadly, clients don't want to "spend money" on Travis-CI for their private repos ... 😢 💸 So we have adopted Circle-CI for those projects ... @des-des kindly started writing: https://github.com/dwyl/learn-circleci however there is still a lot of knowldege that remains tacit ... 😕

Tasks/Todo:

• [x] Expand/complete the Circle-CI Tutorial
• [x] explain that we only use it for private projects (because Travis-CI is bettererer!)
• [x] add section issues we have had to overcome while using Circle-CI (share your learnings!)
• [ ] add to line to: https://github.com/dwyl/technology-stack see: https://github.com/dwyl/technology-stack/issues/25

[nelsonic]

I have summarised the process for CI/CD. Deployment to a Dokku PaaS on DO. Closing.