[Docs] Document how to upload posters

docs/user/00_index.rst

-Publishing time-based documents with Resint
-===========================================
+Time-based documents with Resint
+================================
 
 .. toctree::
    :glob:

docs/user/01_posters.rst

+Publishing posters
+==================
+
+Posters are documents that can be manually supplied in a
+time-based manner.
+
+Poster files can be uploaded for a defined time period,
+and AlekSIS will then deliver the currently valid version
+under a stable menu item and URL.
+
+Posters are organised in psoter groups, which can be
+created by administrators. Users can then upload PDF files
+which shall be published for each time period.
+
+Users who have the permission to upload posters for at
+least one poster group can manager their psoters from the
+main menu under the `Documents → Manage posters` menu item.
+
+.. image:: ../_static/manage_posters_list.png
+  :width: 600
+  :alt: Manage posters
+
+Using the `Upload poster` button or the `Edit` action for
+an already uploaded poster will open the dialog for manaing
+a document for one time period.
+
+.. image:: ../_static/upload_poster.png
+  :width: 600
+  :alt: Upload poster
+
+After uploading, AlekSIS will deliver the uploaded file
+during the week selected in the form.