Add changelog commentary
Version 3.12.0
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+This version has two major changes:
+
+* It updates our default docker images to stable=5.0 and latest=6.0.
+* It changes our PR builder domain to `readthedocs.build`
+
:Date: February 18, 2020
* `@humitos <https://github.com/humitos>`__: Use PUBLIC_DOMAIN_USES_HTTPS for resolver tests (`#6673 <https://github.com/readthedocs/readthedocs.org/pull/6673>`__)
