Add preliminiary text for import format

Just as a placeholder for translation once the UI is finished

src/views/users.html

@@ -351,6 +351,7 @@
       <div class="modal-body">
         <div ng-show="!userImport.done">
           <div ng-show="!userImport.busy">
+            <p>The import requires a specific schema for both JSON and CSV. The detailed schema is described in our <a href="" target="_blank">documentation</a></p>
             <input type="file" style="display: none;" id="userImportFileInput" accept="application/json,text/csv"/>
             <button class="btn btn-primary" ng-click="userImport.openFileInput()">Select JSON or CSV file</button>
             <br/>