fix: include instructions for append in system prompt

gptme/tools/patch.py

@@ -16,6 +16,7 @@
 
 The LLM agent can patch files, by using a adapted version git conflict markers.
 This can be used to make changes to files we have written in the past, without having to rewrite the whole file.
+We can also append to files by prefixing the filename with `append`.
 
 ## Example
 
@@ -34,6 +35,13 @@ def hello():
     print(f"hello {name}")
 >>>>>>> UPDATED
 ```
+
+> User: run the function when the script is run
+> Assistant:
+```append hello.py
+if __name__ == "__main__":
+    hello()
+```
 """