Add release notes for UsernameField support

Author: Afonso Cerejeira

hypothesis-python/RELEASE.rst

@@ -0,0 +1,8 @@
+RELEASE_TYPE: minor
+
+:func:`~hypothesis.extra.django.from_field` now supports
+:class:`~django.contrib.auth.forms.UsernameField` and
+:class:`~django.contrib.auth.forms.ReadOnlyPasswordHashField`
+from ``django.contrib.auth.forms``.
+
+Thanks to Afonso Silva for reporting and fixing :issue:`3417`.