Adds comment about not changing or creating other language files manually.

lang/main.json

+/**
+ * Object with translatable texts for Jitsi Meet in English.
+ *
+ * WARNING: The files for the other languages must not be created or changed manually.
+ * They are generated by Pootle and the translation must be performed in the Pootle user interface on http://translate.jitsi.org.
+ * If you're missing a language or have other questions, please send an e-mail to dev@jitsi.org
+ */
 {
     "contactlist": "CONTACT LIST",
     "connectionsettings": "Connection Settings",