Encourage MR author preparation in Code Review Guidelines documentation

What does this MR do?

Encourages MR author preparation before review, based on findings in https://www.ibm.com/developerworks/rational/library/11-proven-practices-for-peer-review/index.html#__RefHeading__97_174136755.

Slack conversation: https://gitlab.slack.com/archives/C02PF508L/p1543153967824700

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards

/label ~Documentation