Add README documenting optional transform families and new NeutralAD flags

[denix56]

Motivation

• Provide a concise top-level description of the three optional transform families and the new configuration flags added to NeutralAD so users know how to enable and configure them.

Description

• Add README.md at the repository root that documents the three optional transform families (invertible flows, group transforms, and frequency-orthogonal transforms) and the new flags use_invertible_transforms, use_group_transforms, use_freq_ortho_transforms, keep_base_transforms, and transform_families with references to the implementation files.

Testing

• No automated tests were run because this is a documentation-only change (doc-only change).

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Codex Task