Add core guidelines examples

[mokagio]

Now that our core guidelines have been defined, see #1, we should provide examples of how to apply them.

I'd like to see examples of

• Test behaviour not implementation
• Fast
• Isolated
• Self-verifying
• Tests should read like sentences
• One assertion per test
• AAA

I think fast and isolated are quite related with each other, so they might have an example together.

In the case of "test behaviour not implementation" I'd suggest having a preferred vs not preferred example, to make it clearer what the difference is.

[marcelmika]

What do you think about using the banana example for the "Test behavior, not implementation": https://github.com/Quick/Quick/blob/master/Documentation/en-us/BehavioralTesting.md

I can create some other but I think this is quite sufficient.

[mokagio]

I think that example is great and definitely could be added as a "further reading". From my point of view it's too long though. Also the fact that it looks into a database will result a bit confusing later on when we'll suggest to use a test double for such things.