docs(CHANGES): Note pytest-xdist

Author: tony

CHANGES

@@ -15,6 +15,30 @@ $ pip install --user --upgrade --pre libtmux
 
 <!-- To maintainers and contributors: Please add notes for the forthcoming version above -->
 
+### Testing
+
+- Add `pytest-xdist` ([PyPI](https://pypi.org/project/pytest-xdist/), [GitHub](https://github.com/pytest-dev/pytest-xdist)) for parallel testing (#522)
+
+  pytest:
+
+  ```console
+  py.test -n auto
+  ```
+
+  pytest-watcher:
+
+  ```console
+  env PYTEST_ADDOPTS='-n auto' make start
+  ```
+
+  entr(1):
+
+  ```console
+  make watch_test test="-n auto"
+  ```
+
+- Flakey tests: Relax `assert` in `retry` tests (#522)
+
 ### Documentation
 
 - Automatically linkify links that were previously only text.