Update changelog

Author: staadecker

REAM Model Changelog.md

@@ -15,4 +15,5 @@ Changes are listed from oldest (first line) to newest (last line of table).
 | #36 | May 2021 | Correct inputs to only list transmission lines in one direction. |
 | #56 | June 2021 | Convert 2020 predetermined build years to 2019 in `get_inputs.py` to avoid conflicts with 2020 period. |
 | #57 | June 2021 | Specify predetermined storage energy capacity in inputs (previously left unspecified). |
-| #68 | June 2021 | Change financial params to 2018 dollars & 5% interest rate. Start using terrain multipliers (which now include the economic multiplier). |
+| #68 | June 2021 | Change financial params to 2018 dollars & 5% interest rate. Start using terrain multipliers (which now include the economic multiplier). |
+| #89 | August 2021 | Change hydro module average flow constraint to a monthly constraint rather than per timeseries. |