OLS-1276: doc about system prompt override mechanism

Signed-off-by: Pavel Tisnovsky <[email protected]>

README.md

@@ -511,6 +511,10 @@ rcs_config:
   system_prompt_path: "system_prompts/system_prompt_for_product_XYZZY"
 ```
 
+Additionally an optional string parameter `system_prompt` can be specified in `/v1/query` endpoint to override the configured system prompt. This override mechanism can be used only when the `dev_config.enable_system_prompt_override` configuration options is set to `true` in the service configuration file. Please note that the default value for this option is `false`, so the system prompt cannot be changed. This means, when the `dev_config.enable_system_prompt_override` is set to `false` and `/v1/query` is invoked with the `system_prompt` parameter, the value specified in `system_prompt` parameter is ignored.
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 # Usage
 
 ## Deployments