Add a story for exporting tests into nitrate

Author: psss

stories/cli/test.fmf

@@ -47,9 +47,22 @@ story: 'As a user I want to comfortably work with tests'
     documented: /docs/examples#convert-tests
 
 /export:
-    story: 'I want to export test metadata into desired format.'
-    example: tmt test export --format yaml
-    implemented: /tmt/base
+    story: 'I want to export test metadata.'
+
+    /format:
+        story: 'I want to export test metadata into given format.'
+        example: tmt test export --format yaml
+        implemented: /tmt/base
+
+    /nitrate:
+        story: 'I want to export test metadata into nitrate.'
+        example: tmt test export --nitrate
+        description: |
+            In order to keep metadata in sync with the old test
+            case management system we need to export selected set
+            of attributes (tags, contact, relevancy) back to
+            nitrate. The full fmf object identifier should be
+            added to the structured field under the key ``fmf``.
 
 /coverage:
     story: 'Test coverage overview'