Add reference to main repo for make command

Author: Flamefire

README.md

@@ -28,7 +28,8 @@ Language files for s25client
     - Check and fix auto-translations
     - Remove unused translations (Validate, that they are actually unused)
     - Save
-    - Run `make -B translations` in the build directory to have gettext sort the translations.  
+    - Run `make -B translations` in the build directory of RTTR to have gettext sort the translations.
+      Note that you need to have [RTTR](https://github.com/Return-To-The-Roots/s25client) configured already for that to work.  
       Alternatively run `msgmerge --sort-output --no-wrap --quiet --update --backup=none <abspath-to-file>.po <abspath-to-rttr.pot>` on every po-file.
     - commit
 - Create a pull request with the change