Add release notes for 0.15 release

docs/ref/release_notes.rst

@@ -1,6 +1,47 @@
 Release Notes
 =============
 
+What's new in iris-grib v0.15
+-----------------------------
+
+:Release: 0.15.0
+:Date: 5 Dec 2019
+
+Features
+^^^^^^^^
+
+* Updated translations between GRIB parameter code and CF standard_name or
+  long_name :
+      * additional WAFC codes, both to and from CF
+      * 'mass_fraction_of_cloud_liquid_water_in_air' and 'mass_fraction_of_cloud_ice_in_air', both to and from CF
+      * 'surface_downwelling_longwave_flux_in_air', now translates to GRIBcode(2, 0, 5, 3)  (but not the reverse).
+      * for full details, see : https://github.com/Scitools/iris-grib/compare/c4243ae..5c314e3#diff-cf46b46880cae59e82a91c7ab6bb81ba
+
+* Added support for loading GRIB messages with no fixed surface set in the
+  product definition section
+
+* Added support for loading GRIB messages where i or j increment are not set
+
+* Added support for saving cubes that have a "depth" coordinate
+
+* Cubes loaded from GRIB files now contain a new GRIB_PARAM attribute, the
+  value of which is an instance of
+  iris_grib.grib_phenom_translation.GRIBCode and represents the parameter code.
+  When saving, if a cube has a GRIBCode attribute, this determines the parameter code
+  in the created message(s): This will _override_ any translation from the CF names.
+
+Bug Fixes
+^^^^^^^^^
+
+* Reverted a bug that was fixed in v0.13 related to loading hybrid pressure 
+  levels. It was agreed that the initial behaviour was correct 
+
+Dependencies
+^^^^^^^^^^^^
+
+* Python 2 is no longer supported
+
+
 What's new in iris-grib v0.14
 -----------------------------