Add announcement about import attrs

Author: hynek

changelog.d/487.deprecation.rst

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+The long-awaited, much-talked-about, little-delivered ``import attrs`` is finally upon us!
+
+Since the NG APIs have now been proclaimed stable, the **next** release of ``attrs`` will allow you to actually ``import attrs``.
+We're taking this opportunity to replace some defaults in our APIs that made sense in 2015, but don't in 2021.
+
+So please, if you have any pet peeves about defaults in ``attrs``'s APIs, *now* is the time to air your grievances in #487!
+We're not gonna get such a chance for a second time, without breaking our backward-compatibility guarantees, or long deprecation cycles.
+Therefore, speak now or forever hold you peace!