Update README.md

Add prompt template for the scenario where context is not available.

README.md

@@ -40,6 +40,8 @@ Note that ChatQA-1.5 is built based on Llama-3 base model, and ChatQA-1.0 is bui
 
 
 ## Prompt Format
+**We highly recommend that you use the prompt format we provide, as follows:**
+### when context is available
 <pre>
 System: {System}
 
@@ -54,6 +56,22 @@ User: {Question}
 Assistant:
 </pre>
 
+### when context is not available
+<pre>
+System: {System}
+
+User: {Question}
+
+Assistant: {Response}
+
+User: {Question}
+
+Assistant:
+</pre>
+**The content of the system's turn (i.e., {System}) for both scenarios is as follows:**
+<pre>
+"This is a chat between a user and an artificial intelligence assistant. The assistant gives helpful, detailed, and polite answers to the user's questions based on the context. The assistant should also indicate when the answer cannot be found in the context."
+</pre>
 
 ## How to use