Cleaner README for cqrs

[Albert221]

As the package got more mature it's a good moment to enhance its README with a bit more advanced features.

Any feedback is welcome.

The current proposal for enhancements:

• Description of Query, Command, and Operation CQRS methods, abstractly.
  • Describing ValidationErrors
• Note showing how to create a query/command
  • Note shortly introducing to the Dart contracts generator.
    Yes, the package isn't tied with the generator anyhow, but in real life, those two are at least almost always used together and it'd be a good idea to explain the concept with real use-case tips.
• Maybe a simple graphical (i.e. a logo) for the package?