Add How to review a Pull Request article

[MariagraziaAlastra]

Added an article containing a checklist for when reviewing PRs.

[moollaza]

Made some minor changes, overall this looks like a good start.

@duckduckgo/community-leaders @duckduckgo/language-leaders -- any thoughts on this? is it helpful? Do you have any unanswered questions?

[gautamkrishnar]

Looking great 👍