Clarity on Step 6 - nodejs-pm2-zero-downtime.md

Would someone be able to provide better clarity on [Step #6](https://github.com/dwyl/learn-devops/blob/master/nodejs-pm2-zero-downtime.md#6-add-deployment-sshrsa-public-key)?

I am confused in which keys go where / the mentality behind the private+public key sharing. Surprised there's not more explanation on the page to help guide through any common errors. Currently i'm getting a `Permission denied (publickey)` error when i run `pm2 deploy ecosystem.config.js production setup`. Which illustrates that my keys are in the incorrect locations.

Update: This error has been narrowed down to pm2 not being able to run it's deploy scripts. Travis-ci reports that pm2 cannot head that's not pointing to a branch.