Add reviewing checklist to docs/contributions.

There is some overlap with the style guide. Where the style guide aims to be descriptive, this document is simply a list of questions for the reviewer to ask when when reading a PR.
These two lines are inconsisent. `c.Mkdir()` will give you something inside /tmp/gocheck-NNNNNNN, what does the other one do ? Is it easier to say "temporary files must be created inside a path provided by `c.Mkdir`"