Add style guidelines on squashing and merging

* Merge commits add noise the Git log.
* A tree-based history is harder to follow than a linear log.
* Encourages small, cohesive commits.

Some of this is already covered in Protocl and Best Practices, but I
thought it would be good to have something here related to the style
of Git log we want to have.

In particular, I want to avoid "Merged pull request #something" and
"Merged master into somebody/master" commits.