Update README with system prompt environment variable

Added instructions for setting the system prompt via environment variable.

Author: karmaking

README.md

@@ -84,7 +84,11 @@ OBP_CONSUMER_KEY="your-obp-consumer-key"
 ```
 
 ### 5. Changing the system prompt
-Opey only knows about its environment as long as you tell it. The default system prompt is OpenBankProject-focused. Iterating on the prompt for better final responses or better alignment with your particular instance is recommended. All you need to do is edit the [prompt file](/src/agent/components/prompts/opey_system.prompt.yaml)
+Opey only knows about its environment as long as you tell it. The default system prompt is OpenBankProject-focused. Iterating on the prompt for better final responses or better alignment with your particular instance is recommended. The system prompt can be set via the env var
+
+```
+OPEY_SYSTEM_PROMPT
+```
 
 ## Running
 Activate the poetry venv using `poetry shell` in the current directory