Add release notes for multiproc rewrite

Author: aseyboldt

RELEASE-NOTES.md

@@ -13,6 +13,11 @@
 - Improve error message `NaN occurred in optimization.` during ADVI
 - Save and load traces without `pickle` using `pm.save_trace` and `pm.load_trace`
 - Add `Kumaraswamy` distribution
+- Rewrite parallel sampling of multiple chains on py3. This resolves
+  long standing issues when tranferring large traces to the main process,
+  avoids pickleing issues on UNIX, and allows us to show a progress bar
+  for all chains. If parallel sampling is interrupted, we now return
+  partial results.
 
 ### Fixes