Add date to CHANGELOG

Author: josevalim

CHANGELOG.md

@@ -90,7 +90,7 @@ As of Elixir v1.12, implicitly decreasing ranges are soft-deprecated and warning
 
 Elixir v1.12 has the additional of many functions across the standard library. The `Enum` module received additions such as `Enum.count_until/2`, `Enum.product/1`, `Enum.zip_with/2`, and more. The `Integer` module now includes `Integer.pow/2` and `Integer.extended_gcd/2`. Finally, the `Kernel` module got two new functions, `Kernel.then/2` and `Kernel.tap/2`, which are specially useful in `|>` pipelines.
 
-## v1.12.0
+## v1.12.0 (2021-05-19)
 
 ### 1. Enhancements