add changelog entry for fix (#553)

diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 263c724..b86211d 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -23,6 +23,8 @@
 ^^^^^^^^
 
 - Fixed compatibility errors with OpenSSL 1.1.0.
+- Fixed an issue that caused failures with subinterpreters and embedded Pythons.
+  `#552 <https://github.com/pyca/pyopenssl/pull/552>`_
 
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