Want to add information about set :commit

[coderhs]

We recently had to deploy a previous version of our project. We found that mina had ability to deploy a particular git commit with it by looking through the code. I would like to document that setting, but not sure which section to add it. Could someone point me in the right direction and I will send the PR.

https://blog.redpanthers.co/using-mina-deploy-particular-commit/