Add changelog entries for asyncio support

Author: Hugo Osvaldo Barrera

CHANGELOG.rst

@@ -15,12 +15,20 @@ Version 0.19.0
 - Add "description" and "order" as metadata.  These fetch the CalDAV:
   calendar-description, CardDAV:addressbook-description and apple-ns:calendar-order
   properties.
-- Add a new "showconfig" status. This prints *some* configuration values as
+- Add a new ``showconfig`` status. This prints *some* configuration values as
   JSON. This is intended to be used by external tools and helpers that interact
   with ``vdirsyncer``.
-- Update TLS-related tests that were failing due to weak MDs.
+- Update TLS-related tests that were failing due to weak MDs. :gh:`903`
 - ``pytest-httpserver`` and ``trustme`` are now required for tests.
 - ``pytest-localserver`` is no longer required for tests.
+- Multithreaded support has been dropped. The ``"--max-workers`` has been removed.
+- A new ``asyncio`` backend is now used. So far, this shows substantial speed
+  improvements in ``discovery`` and ``metasync``, but little change in `sync`.
+  This will likely continue improving over time. :gh:`906`
+- Support for `md5` and `sha1` certificate fingerprints has been dropped. If
+  you're validating certificate fingerprints, use `sha256` instead.
+- The ``google`` storage types no longer require ``requests-oauthlib``, but
+  require ``python-aiohttp-oauthlib`` instead.
 
 Version 0.18.0
 ==============