fix: adding descripition on readme file about the new environment var…

…iable

README.md

@@ -130,6 +130,17 @@ the `BP_NODE_PROJECT_PATH` environment variable at build time either directly
 file](https://github.com/buildpacks/spec/blob/main/extensions/project-descriptor.md).
 This could be useful if your app is a part of a monorepo.
 
+### Setting explicitly a run image `BP_NODE_RUN_EXTENSION`
+
+With `BP_NODE_RUN_EXTENSION` environment variable, you are able to specify the run image of the built application, without changing the source code of the extension (specifically the extension.toml file) as shown on below example.
+
+```bash
+  pack build test-app-name \
+     --path ./app-dir \
+     --builder paketocommunity/builder-ubi-base \
+     --env BP_NODE_RUN_EXTENSION="localhost:5000/my-run-image"
+```
+
 ## Run Tests
 
 To run all unit tests, run: