Add some documentation about cron jobs

Fixes https://pagure.io/pagure/issue/2053

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

doc/index.rst

@@ -38,6 +38,7 @@ Contents:
    install_pagure_ci
    install_pagure_loadjson
    install_pagure_logcom
+   install_crons
    configuration
    custom_gitolite_conf
    development

doc/install_crons.rst

@@ -0,0 +1,28 @@
+Cron Jobs
+=========
+
+Some actions in pagure are meant to the run via a cron job.
+
+
+API key expiration reminder
+---------------------------
+
+One of the cron job sending reminder about API keys that are about to expire.
+It will send an email 10 days, then 5 days and finally the day before the
+key expires to the person who has created.
+
+The cron job can be found in the sources in: ::
+
+    files/api_key_expire_mail.py
+
+In the RPM it is installed in: ::
+
+    /usr/share/pagure/api_key_expire_mail.py
+
+This cron job is meant to be run daily using a syntax similar to:
+
+::
+
+    10 0 * * * root python /usr/share/pagure/api_key_expire_mail.py
+
+which will make the script run at 00:10 every day.