clarify transifex usage

Signed-off-by: tobiasKaminsky <tobias@kaminsky.me>

CONTRIBUTING.md

@@ -156,6 +156,10 @@ With this approach the "downtime" of not building master is limited to the times
 ## Translations
 We manage translations via [Transifex](https://www.transifex.com/nextcloud/nextcloud/android/). So just request joining the translation team for Android on the site and start translating. All translations will then be automatically pushed to this repository, there is no need for any pull request for translations.
 
+If you need to change a translation, do not change it, but give it new key. This way the translation stays backward compatible as we automatically backport translated strings to last versions.
+
+When submitting PRs with changed translations, please only submit changes to values/strings.xml and not changes to translated files. These will be overwritten by next merge of transifex-but and increase PR review.  
+
 # Releases
 At the moment we are releasing the app in two app stores: