Add test and docs

docs/image_information.rst

@@ -6,6 +6,7 @@ Additional information on an image can be given in a file using the `markdown`_
 syntax, named ``<imagename>.md`` (example: ``IMG_5206.md``)::
 
     Title: My awesome photo
+    Date: 2020-01-01T09:00:00
 
     And a description with *Markdown* syntax.
 
@@ -13,6 +14,8 @@ EXIF data is directly extracted, see :ref:`simple-exif-data`. Some meta-data
 keys are used by Sigal to get the useful informations on the gallery:
 
 - *Title*: the image title.
+- *Date*: the file date, useful when it cannot be read from the EXIF metadata,
+  e.g. for videos and some image formats.
 
 Any additional meta-data is available in the templates. For instance::