Add to README about compiling JS production assets.

Add note in README about the need to checkin also compiled JS assets
after running `make build-js-production`.

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

README.md

@@ -89,6 +89,13 @@ Then inside the root folder of your local Nextcloud development installation, ru
 ./build/compile-handlebars-templates.sh
 ```
 
+Before checking in JS changes, make sure to also build for production:
+```
+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.
 
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