Add basic code review guidelines

[OleksiyRudenko]

@AMashoshyna A suggestion for your consideration on a section to add.

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Check 2 or 3 random PRs from the peer repo as reference examples of code review. Take a note of what mentors suggest to improve.

• JS DOM
• Building a Tiny JS World
• OO JS / Frogger
• OOP Exercise / TJSW OOP
• Memory Pair Game
• Friends App

PRs with irrelevant commits can also be a source of ideas on how to support a student in issue resolution.

[OleksiyRudenko]

Not sure where to put these technical aspects of PR/code review:

• https://lab.github.com/githubtraining/reviewing-pull-requests
• https://help.github.com/articles/about-pull-request-reviews/
• https://help.github.com/articles/reviewing-proposed-changes-in-a-pull-request/