update release notes

hypothesis-python/RELEASE.rst

@@ -1,3 +1,11 @@
 RELEASE_TYPE: minor
 
-FILL ME IN LATER
+Since :ref:`v6.27.1 <_v6.27.1>` the backing data structure of 
+:func:`~hypothesis.register_random` is a :class:`weakref.WeakKeyDictionary`. As a 
+consequence, passing an unreferenced object to :func:`~hypothesis.register_random` will 
+have no affect on Hypothesis' tracking of RNG sources. This patch modifies 
+:func:`~hypothesis.register_random` to raise an error when it is passed an unreferenced 
+object, and to emit a warning when it looks like it was passed an object that is only 
+referenced within a temporary scope. These checks are not performed by PyPy's
+interpreter. The type annotation of :func:`~hypothesis.register_random` was also 
+widened to permit structural subtypes of ``random.Random``.