Add DAQ Policy documentation

source/daq_policy.rst

+DAQ Policy
+==========
+Not every parameter of a device is interesting to record, such as the provided scenes.
+As such, the policy for each individual property can be set, on a per-instance basis.
+
+These are specified using the :class:`DAQPolicy` enum:
+
+ - `OMIT`: will not record the property to file;
+ - `SAVE`: will record the property to file;
+ - `UNSPECIFIED`: will adopt the global default DAQ policy. Currently, it is set to
+   record, although this will eventually change to not recorded.
+
+Legacy devices which do not specify a policy will have an `UNSPECIFIED` policy set
+to all their properties.
+
+.. note::
+    This are applied to leaf properties. Nodes do not have DAQPolicy.
+
+Developers should liaise with users to define which properties should be recorded.
+These can be set up programmatically:
+
+.. code-block:: c++
+
+   using namespace karabo::util;
+
+   void MyDevice::expectedParameters(karabo::util::Schema& expected) {
+           UINT32_ELEMENT(expected).key('enzymAssay')
+               .displayedName("Assay")
+               .description("Enzym Activity")
+               .unit(Unit::KATAL)
+               .metricPrefix(MetricPrefix::MICRO)
+               .daqPolicy(DAQPolicy::SAVE)
+               .readOnly().initialValue(0)
+               .commit();
+       }
+
+The policy can be overwritten from the GUI, before instantiation, and saved in a project:
+
+.. image:: graphics/daq_policy.png