docs: add email client compatibility notes

[Shreyans-704]

Summary

Adds email client compatibility documentation requested in #23.

Changes

• Documents the primary target clients: Outlook, Gmail, Yahoo, and Apple Mail.
• Explains why email mode uses table-based HTML and Outlook/MSO-specific patterns.
• Documents repository-scoped compatibility caveats and clarifies that automated validation is not an inbox-client emulator matrix.
• Documents the CSP safety probe and its strict-CSP interpretation.
• Links the compatibility notes to the relevant snapshot, golden-template, browser E2E, and CSP validation files.
• Preserves the existing Litmus and Email on Acid validation information.

Validation

• pnpm test — 450 tests passed.
• git diff --check — clean.
• pnpm test:csp — currently fails because of a React CommonJS/ESM interop issue (createContext named export), not because of a CSP code-generation failure.
• No source code, tests, dependencies, or generated snapshots were changed.

Closes #23

[Copilot]

Pull request overview

Adds a new “Email Client Compatibility Notes” section to the root README to make Elements’ email-client targeting, rationale (table/MSO patterns), validation scope, and CSP gate expectations explicit for evaluators—aligning with the documentation request in #23.

Changes:

• Documented the primary targeted email clients (Outlook, Gmail, Yahoo, Apple Mail).
• Explained why <Email> renders table-based / MSO-specific markup and linked to repo evidence and validation tests.
• Documented the strict CSP probe intent and how to interpret test:csp results.

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