Clarify description of push field (fixes #336)

trans/models/subproject.py

@@ -89,7 +89,7 @@ class SubProject(models.Model, PercentMixin, URLMixin, PathMixin):
     push = models.CharField(
         verbose_name=ugettext_lazy('Git push URL'),
         max_length=200,
-        help_text=ugettext_lazy('URL of push Git repository'),
+        help_text=ugettext_lazy('URL of push Git repository, pushing is disabled if empty.'),
         blank=True
     )
     repoweb = models.URLField(