add a reference to the jekyll.workflow project

I wanted to add a reference to [jekyll.workflow](https://github.com/gtramontina/jekyll.workflow), a simple set of scripts I've put together that makes deploying to `gh-pages` a one command task.
Initially I thought of adding this to the `github-pages.md` documentation, but then it looked too "GitHub-official", that's why I added to `deployment-methods.md`, as it seems more community-driven.

Cheers,
- g