Document what a good commit series looks like (a story)

[mcomella]

A wrote this at one point:

While you can use commits for yourself, a commit series in a pull request is for the reviewer (or the bisector): they should tell a story which helps the reviewer understand the code. A good story should be modular and shouldn't go back on what it has already said.

For example, a good story says "I put down this building block and this building block so I can place another building block on top of both them, which makes it easier to fit this building block, and the final building block on top of that, which completes the issue." However, this story feels like, "I put down this building block, then picked it up and moved it back here, then added another building block, which I moved forward an inch, ..."

[Aman0307]

@mcomella I would like to contribute to this issue. Please let me know.