Merge Strategy: Add some guidance on when to use/ pros & cons of using --no-ff versus squash and merge

[adamsilverstein]

When working on improving recommendations for deleting and potentially archiving branches in #271 it became clear that the best approach depends on the merge strategy the project is using. @nicholasio suggested we consider "adding some guidance on when to use --no-ff versus squash and merge" #271 (review) resulting in this issue.