Update README.md

[erfanium]

In this draft PR, I want to improve the docs. There are some issues that I see

• Nullable and default values missing doc.
• [ x] Explain how fastest-validator is fast and what v.compile really do and why you should not call it every time.
• Remove v.validate usage from docs.

[icebob]

Are you working on it, yet?

[erfanium]

No,
I'll probably finish it in the next two days