Add note the ACP needs amp credit and not work on free credit

Author: Shashikant86

README.md

@@ -83,6 +83,11 @@ No installation needed — npx downloads and runs automatically!
 
 Before using acp-amp, you need:
 
+0. **Paid Amp credits required**:
+   - ACP access requires a paid Amp credit balance.
+   - The free credit version does not work for ACP.
+   - Top up a few dollars in Amp to include the ACP functionality.
+
 1. **Amp CLI** installed and authenticated:
    ```bash
    curl -fsSL https://ampcode.com/install.sh | bash

docs/index.md

@@ -19,6 +19,8 @@
 
 acp-amp bridges [Amp Code](https://ampcode.com) to the [Agent Client Protocol (ACP)](https://github.com/agentclientprotocol/agent-client-protocol), allowing you to use Amp's powerful AI coding capabilities in any ACP-compatible client.
 
+**Important:** ACP requires a paid Amp credit balance. The free credit version does not work for ACP. Top up a few dollars in Amp to enable ACP access.
+
 **Available in two versions:**
 
 | Version | Install | Best For |