Add requested merge style to CONTRIBUTING.md

[michaelmhoffman]

I would merge #196 but I do not know whether merge commit, squash and merge, or rebase and merge are preferred. Please specify in CONTRIBUTING.md.

[agitter]

@Benjamin-Lee you've been using merge commits in this repository, right?

Regardless of what we specify as the preferred merged style for general updates, we need to specify that rootstock updates are made via merge commits.

[dhimmel]

Personally, I haven't found much user for rebase commits, since usually I'd like to make the merging explicit (since master branch CI will only run on the tip rebase commit IIRC). As @agitter mentioned, merge should be used for manubot updates pulled from rootstock. Otherwise, it's up to project maintainers. I usually squash unless there is some reason to keep the composite commits (like multiple authors or to break up a large changeset).