Docs: Add Email Client Compatibility Notes

[ericdellacasa8]

Context

Developers evaluating an email rendering library usually want to know which email clients are supported and what caveats exist.

Elements mentions support for major email clients, but does not currently document compatibility expectations clearly.

Definition of Done

Add an email client compatibility section to the root README or documentation.

The section should:

• List the main targeted clients, such as Outlook, Gmail, Yahoo, and Apple Mail
• Explain that email mode is table-based for email client compatibility
• Mention any known caveats or limitations
• Mention the CSP-safe rendering guarantee
• Link to relevant tests or internal validation where appropriate

Example Outcome

A developer evaluating Elements should be able to quickly understand:

• Which email clients Elements is designed to support
• Why the email output is structured the way it is
• Any known limitations they should be aware of before adopting it

This should make the repo easier to evaluate for email-focused developers.

Where to Start

Useful references already exist in the codebase:

• elements/CLAUDE.md
• packages/react/scripts/csp-probe.mjs
• packages/react/src/components/snapshots.test.tsx

The goal is to turn the existing compatibility assumptions and validation work into clear public documentation.