Update documentation with mdq freshness_period

Author: nsklikas

docs/howto/config.rst

@@ -209,6 +209,7 @@ For example::
             {
                 "url": "http://mdq.ukfederation.org.uk/",
                 "cert": "ukfederation-mdq.pem",
+                "freshness_period": "P0Y0M0DT2H0M0S",
             },
         ],
     },
@@ -221,6 +222,14 @@ metadata signing certificates should be used.  These public keys must be
 acquired by some secure out-of-band method before being placed on the local
 file system.
 
+When using MDQ, the `freshness_period` option can be set to define
+a period for which the metadata fetched from the the MDQ server are considered
+fresh. After that period has passed the metadata are not valid anymore and must
+be fetched again. The period must be in the format defined in `iso 8601 <https://www.iso.org/iso-8601-date-and-time-format.html>`_.
+
+By default, if `freshness_period` is not defined, the metadata are fetched
+only the first time they are requested and never refreshed.
+
 organization
 ^^^^^^^^^^^^