docs: add LLM provider documentation with pricing details

Add comprehensive documentation for budget-friendly LLM providers:
- Z.ai coding plans (Lite, Pro, Max tiers)
- OpenRouter free tier limits and pricing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -1,2 +1,41 @@
-# penny-pincher-provider
-Penny-Pincher Provider, list of awesome cheap (or almost free) LLM provider that suitable for people who unwilling to pay for AI
+# Penny-Pincher Provider
+
+Penny-Pincher Provider, list of awesome cheap (or almost free) LLM provider that suitable for people who unwilling to pay for AI.
+
+*If you find this helpful, you can support me by donate or just register with my referral link. Thank you*
+
+## Providers support coding plans
+
+These provider provide a coding plan subscription, you can pre-pay a fee each month, then use their LLM api with a limit
+
+### [Z.ai](https://z.ai/subscribe)
+
+Offer 3 plans:
+1. Lite: 3x usage of the Claude Pro plan
+2. Pro: 5x Lite plan usage
+3. Max: 4x Pro plan usage
+
+*Prices may change vary so I don't include here*
+
+Source: https://z.ai/subscribe [^z.ai]
+
+Homepage: https://docs.z.ai/devpack/overview
+
+My referral:
+
+>🚀 You've been invited to join the GLM Coding Plan! Enjoy full support for Claude Code, Cline, and 20+ top coding tools — starting at just $10/month. Subscribe now and grab the limited-time deal! Link： https://z.ai/subscribe?ic=PLKIAYEIPW
+
+[^z.ai]: Check at Mar 25 2026
+
+## Free providers
+
+### [OpenRouter](https://openrouter.ai)
+
+> Free usage limits: If you’re using a free model variant (with an ID ending in :free), you can make up to 20 requests per minute. The following per-day limits apply:
+> - If you have purchased less than 10 credits, you’re limited to 50 :free model requests per day.
+> - If you purchase at least 10 credits, your daily limit is increased to 1000 :free model requests per day.
+
+Source: https://openrouter.ai/docs/api/reference/limits [^openrouter]
+[^openrouter]: Check at Mar 25 2026
+
+Homepage: https://openrouter.ai