[docs] terminology section

[pcarbonn]

I would find it very useful to have a terminology section. It would define, at a minimum, what is a branch, a patch, a (stage/unstage) change and a commit, and the links between them. These concepts are not yet clear to me.

Side note: the tutorial says: "This tutorial assumes familiarity with the basics of Git, including commits, branches, and merge conflicts." Shouldn't stg target users that do not have that familiarity ? If you are already good at git, why bother with stg ?