Things to improve on the Docs

[gustavoguichard]

I'm gonna add a few items here and ideas are welcome.

• Explain differences between fieldError, error, and globalError. From [Docs: explain differences between fieldError, error, and globalError #50]
• Explain why we can't use defaultValue
• Add a custom select input example? [Could it be worth adding a Custom Select input example? #249]