Add changelog entry for MersenneTwisterEngine updates

Author: WebDrake

changelog/std-random-MersenneTwisterEngine.dd

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+`MersenneTwisterEngine` has been updated so that its template signature
+matches the C++11 standard (adding two new template parameters, an extra
+tempering parameter `d` and the initialization multiplier `f`).
+
+For anyone using the standard template instantiation `Mt19937` this will
+have no noticeable effect.  However, this will be a breaking change for
+anyone using the `MersenneTwisterEngine` template directly.
+
+The internal implementation has been reworked to use Ilya Yaroshenko's
+highly optimized algorithm from `mir.random`.  This should have a very
+noticeable positive effect for anyone who cares about generating a lot
+of random numbers quickly.
+
+A new `Mt19937_64` template instantiation has been added, corresponding
+to the standard 64-bit implementation of the algorithm (MT19937-64).
+This fixes $(LINK https://issues.dlang.org/show_bug.cgi?id=10900).