docs: clarify Wrapf deprecation and fmt.Errorf Go 1.13+ wrapping

[AdamMagued]

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Description

In errwrap.go, the docstring for Wrapf previously stated that users should replace fmt.Errorf with Wrapf, while directly below marking Wrapf as Deprecated: Use fmt.Errorf().

Additionally, README.md noted that wrapping with fmt.Errorf loses error structure, and used deprecated errwrap.Wrapf in the basic usage example.

This change:

• Clarifies in Wrapf docstring that Go 1.13+ natively supports error wrapping via fmt.Errorf("%w", ...).
• Removes the outdated recommendation to replace fmt.Errorf with Wrapf.
• Updates README.md to note that prior to Go 1.13 wrapping via fmt.Errorf lost error structure, while Go 1.13+ supports native wrapping via fmt.Errorf("%w", ...).
• Updates the basic usage example in README.md to use fmt.Errorf("%w", ...) instead of deprecated Wrapf.

Fixes #12

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