prep changelog for v2.4

Author: rsyring

CHANGES.rst

@@ -1,3 +1,21 @@
+Version 2.4.0
+-------------
+
+Unreleased
+
+- Make SA engine configuration more flexible (`#684`_)
+- Address SQLAlchemy 1.3 deprecations (`#684`_)
+- `get_or_404()` and `first_or_404()` now accept a description param (`#636`_)
+- Windows: replace time.clock with perf_counter for py3 (`#638`_)
+- Improve readme, docs, tox and update Python versions in CI (`#687`_)
+- Dropped support for Python 2.6, 3.3 and added 3.7 support (`#687`_)
+
+.. _#684: https://github.com/pallets/flask-sqlalchemy/pull/684
+.. _#636: https://github.com/pallets/flask-sqlalchemy/pull/636
+.. _#638: https://github.com/pallets/flask-sqlalchemy/pull/638
+.. _#687: https://github.com/pallets/flask-sqlalchemy/pull/687
+
+
 Version 2.3.2
 -------------