Update contributor's guide, add review guide

[deepthi]

Our contributor's guide is at https://vitess.io/docs/contributing/ We don't have a review guide, we need to add one. In the meantime let us capture improvements to contributor's guide and what we want in the review guide here.

Contributor guide improvements

• Add a walkthrough example of how to write a unit test and how to run it. The plan is to take a good first issue and explain all the steps needed to fix it.

Review guidelines

• For a bug fix, PR description should either include a link to an issue that describes the bug OR an actual description of the bug and how to reproduce + a description of the fix
• For a new feature, a link to a Feature request Issue or RFC that documents use cases, corner cases and test cases.
• unit or endtoend tests required for bugfixes
• vtctl command output order should be stable and awk-able
• check and require docs for new features
• for non-trivial changes code should have some comments as to why things are implemented the way they are
• protobuf changes should be wire-compatible
• changes to _vt tables need to be +/-1 version compatible
• If a new flag is being introduced, the name of the flag should be clear and intuitive (as far as possible), and the flag help should be descriptive. Is it really needed?
• RPC changes need to be +/-1 version compatible
• validate that PR has the correct release notes label

[mhmohona]

Hello @deepthi, is anything mentioned in issue description needs to be done? I want to work on it.