Mention security in changelog
Version 5.19.0
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+.. warning:: This release contains a security fix to our CSRF settings: https://github.com/readthedocs/readthedocs.org/security/advisories/GHSA-3v5m-qmm9-3c6c
+
:Date: June 15, 2021
* `@stsewd <https://github.com/stsewd>`__: Builds: check for nonexistent object (`#8261 <https://github.com/readthedocs/readthedocs.org/pull/8261>`__)
