docs: add GetXAPI pairing recipe for cheaper Twitter read operations

[bozad]

This adds a short "Pairing with GetXAPI for Cheaper Read Operations (Optional)" section to the README.

What this PR does

• Documents a side-by-side integration pattern: keep this project for its primary workflow, optionally pair with GetXAPI for cheaper read operations (search, profile, follower lookups).
• Links to a reference PR pattern in a sibling project (feat: add optional GetXAPI backend for search_tweets GenAIwithMS/twitter-mcp#3) for users who want to fork and add a backend toggle at the code level.
• Notes the pairing is fully optional, default behavior unchanged.

Why this is useful

The official Twitter / X API basic tier is $200 / month for limited reads. For AI agents that need to ingest a lot of tweet history (research, OSINT, monitoring), the cost adds up. GetXAPI prices reads at $0.05 per 1K tweets with no per-endpoint rate limits, complementing the workflows in this project.

What this PR does NOT change

• No code changes
• No new dependencies
• No behavior change for existing users
• README only

Related

• Reference PR for code-level backend toggle: feat: add optional GetXAPI backend for search_tweets GenAIwithMS/twitter-mcp#3 (merged)
• Already-merged in this niche (recent reference): docs: add GetXAPI pairing recipe for cheaper Twitter read operations DataWhisker/x-mcp-server#9 and docs: add GetXAPI pairing recipe for cheaper Twitter read operations hridaydutta123/awesome-twitter-tools#37
• GetXAPI MCP server: https://github.com/getxapi/getxapi-mcp
• GetXAPI signup with $0.50 free credit: https://getxapi.com/signup

Happy to adjust wording or remove anything that does not fit the project voice.