Adds justification for tutorial guideline

[eliotjordan]

Closes #50

[jeremyf]

Does an examples directory count as a tutorial? An example of these examples includes:

• https://github.com/dry-rb/dry-validation/tree/master/examples
• https://github.com/JEG2/highline/tree/master/examples

[afred]

@jeremyf I'd say yes as long as it's noted somewhere in the README or other documentation.