Enhance README to mention module compilation

Added hint about MODULE=xxx when recompiling production assets.

Signed-off-by: Vincent Petry <vincent@nextcloud.com>

README.md

@@ -95,7 +95,12 @@ make build-js-production
 ```
 Then add the compiled files for committing.
 
-Please note that if you used `make build-js` or `make watch-js` before almost all files changed, so might need to clear the workspace first.
+To save some time, to only rebuild for a specific app, use the following and replace the module with the app name:
+```
+MODULE=user_status make build-js-production
+```
+
+Please note that if you used `make build-js` or `make watch-js` before, you'll notice that a lot of files were marked as changed, so might need to clear the workspace first.
 
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