Add usage principles for developer prompt

- Add usage principles section with clear rules
- Emphasize placing developer prompt only once at session start
- Clarify not to include in user/assistant messages
- Provide correct usage pattern example

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

README.md

@@ -235,6 +235,29 @@ User: "Search for SOL on Solana" Model:
 
 For optimal performance, use the following developer/system prompt when initializing the model:
 
+### Usage Principles (Important)
+
+**Follow these rules for best results:**
+
+1. **Place Once at the Beginning**: Put the developer prompt only once, at the very start of your conversation session
+2. **Do NOT place in user messages**: Never include the developer prompt content in user/assistant messages or tool schemas
+3. **Session-wide persistence**: For multi-turn conversations, keep the same developer prompt at the session start - do not repeat it
+
+**Correct usage pattern:**
+```json
+{
+  "messages": [
+    {"role": "developer", "content": "<developer prompt goes here>"},
+    {"role": "user", "content": "first user query"},
+    {"role": "assistant", "content": "assistant response"},
+    {"role": "user", "content": "second user query"}
+    // No need to repeat developer prompt in subsequent turns
+  ]
+}
+```
+
+### Developer Prompt Content
+
 ```json
 {
   "messages": [