Add documentation

[vzakaznikov]

Update README to contain proper module documentation and then add it to the handbook (https://testflows.com/handbook/).

[vzakaznikov]

Started to work on this #4.

[vzakaznikov]

@Selfeer, here are the suggested further improvements:

• Provide a real-world workflow (first UPDATE then CHECK)
• Snapshots committed to the repo and diffed before updates
• Common pit-falls (missing snapshot, updates that were not reviewed, dynamic output)
• How to clean values before storing
• etc.