doc: add code review guide. (pingcap#3166)

code_review_guide.md

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+# Code Review Guide
+
+## Things to do before you start reviewing the PR
+
+* Make sure you are familiar with the packages the PR modifies.
+
+* Make sure you have enough continuous time to review the PR, use 300 LOC per hour to estimate.
+
+* Make sure you can follow the updates of the PR in the next few work days.
+
+* Read the description of the PR, if it's not easy to understand, ask the coder to improve it.
+
+* For a bug fix PR, if there is no test case, ask the coder to add tests.
+
+* For a performance PR, if no benchmark result is provided, ask the coder to add a benchmark result.
+
+
+## Things to check during the review process
+
+* Am I able to understand the purpose of each unit test?
+
+* Do unit tests actually test that the code is performing the intended functionality?
+
+* Do unit tests cover all the important code blocks and specially handled errors?
+
+* Could procedure tests be rewritten to table driven tests?
+
+* Is the code written following the style guide?
+
+* Is the same code duplicated more than twice?
+
+* Do comments exist and describe the intent of the code?
+
+* Are hacks, workarounds and temporary fixes commented?
+
+* Does this function do more than the name suggests?
+
+* Can this function's behavior be inferred by its name?
+
+* Do tests exist and are they comprehensive?
+
+* Do unit tests cover all the important code branches?
+
+* Could the test code be extracted into a table-driven test?
+
+
+## Things to keep in mind when you are writing a review comment
+
+* Be kind to the coder, not to the code.
+
+* Ask questions rather than make statements.
+
+* Treat people who know less than you with respect, deference, and patience.
+
+* Remember to praise when the code quality exceeds your expectation.
+
+* It isn't necessarily wrong if the coder's solution is different than yours.
+
+* Refer to the code style document when necessary.
+
+
+## Things to remember after you submitted the review comment
+
+* Checkout Github notification regularly to keep track of the updates of the PR.
+
+* When the PR has been updated, start another round of review or give it a LGTM.