README: document default CNI paths and non-standard setup guidance

[66Ton99]

Summary

This updates the README to explicitly document the built-in default plugin/config directories and add guidance for non-standard environments.

Changes

• documents default plugin/config directories used when WithPluginDir and WithPluginConfDir are not provided
• lists defaults for non-Windows and Windows
• adds guidance to explicitly pass directories for rootless or non-standard environments (for example Darwin-based setups)

Why

The defaults are currently implicit in code and easy to miss. Making them explicit in documentation helps users avoid confusing runtime setup issues when system-wide directories are not present or writable.

Review request

I would appreciate review from someone familiar with both go-cni and real-world runtime setups (including Darwin/rootless environments) to confirm this guidance is accurate and helpful.

[66Ton99]

Related context:

• containerd/nerdctl PR: defaults: use user-writable data root on darwin nerdctl#4842
• NixOS/nixpkgs PR: nerdctl: enable darwin and fix default data-root permission issue NixOS/nixpkgs#508463