Convert to CC-BY-3.0

This brings gitref.org in line with other OSS Github Training repos.
Merge pull request #65 from github/cc-by-license

Convert to CC-BY-3.0
Refine dirname: "konnichiwa" is correct as Japanese "good afternoon"
Merge pull request #66 from yaotti/correct-dirname-japanese

Refine dirname: "konnichiwa" is correct as Japanese "good afternoon"
Changed when to once

"Once" is used once an action is complete, while "When" is used when an action begins.
Merge pull request #67 from techy1157/patch-1

Fixed minor grammar error: Changed when to once
Added remark about stashing untracked files
Applied recomennded modifications.
Merge pull request #69 from gergelypolonkai/gh-pages

Added remark about stashing untracked files