Add release notes

Author: dstansby

docs/release.rst

@@ -22,6 +22,20 @@ Breaking changes
   are now keyword only, to support the updated API.
   By :user:`Sam Levang <slevang>`, :issue:`623`
 
+Deprecations
+~~~~~~~~~~~~
+The following ``blosc`` funcitons are deprecated, with no replacement.
+This is because they are not intended to be public API.
+
+- ``numcodecs.blosc.init``
+- ``numcodecs.blosc.destroy``
+- ``numcodecs.blosc.compname_to_compcode``
+- ``numcodecs.blosc.cbuffer_sizes``
+- ``numcodecs.blosc.cbuffer_metainfo``
+
+In addition, ``numcodecs.blosc.decompress_partial`` is deprecated as
+has always been experimental and there is no equivalent in the official
+blsoc Python package.
 
 Fixes
 ~~~~~