add info about DEMetropolis tune changes

Author: michaelosthege

RELEASE-NOTES.md

@@ -4,6 +4,7 @@
 
 ### New features
 - use [fastprogress](https://github.com/fastai/fastprogress) instead of tqdm [#3693](https://github.com/pymc-devs/pymc3/pull/3693)
+- `DEMetropolis` can now tune both `lambda` and `scaling` parameters, but by default neither of them are tuned. See [#3743](https://github.com/pymc-devs/pymc3/pull/3743) for more info.
 
 ## PyMC3 3.8 (November 29 2019)