Add a warning to encourage people to update translations direct… (#1045)

src/locales/README.md

@@ -0,0 +1,5 @@
+Please do not contribute changes directly to these files, as we manage them with Crowdin. Instead:
+
+- to request a new translation, [open an issue](https://github.com/excalidraw/excalidraw/issues/new/choose).
+- to update existing translations, [edit them on Crowdin](https://crowdin.com/translate/excalidraw/10)
+  and we should have them included in the app soon!