Add a changelog

Author: staadecker

REAM Model Changelog.md

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+# REAM Model Changelog
+
+This file specifies the changes that we have done to the model.
+
+Pull requests that change the model in a way that will impact our
+results should list their changes here. Pull requests that don't affect our results 
+(e.g. improvements to performance, refactors, workflow, etc.) should not
+update this file.
+
+## Changes to model
+
+Changes are listed from oldest (first line) to newest (last line of table).
+
+| PR | Change description |
+| ---- | -------------------|
+| #8 | feat: Add support for other types of GHGs (NOx, SO2, CH4) |
+| #12 | fix: use the middle of the period instead of the start as the cutoff for retirement and predetermined buildout. |
+| #36 | fix: Remove duplicate transmission lines |
+| #10 | feat: Add support for California policies module |
+| #40  | feat: Add support for planning reserves module |
+| #50 | Upgraded to Pyomo 6.0 |
+| #56 | fix: Convert 2020 predetermined build years to 2019 in get_inputs to avoid conflicts with 2020 period. |
+| #57 | fix: specify predetermined storage energy capacity (previously left unspecified). |
+| #42 | feat: Add parameters to the storage module for self-discharge rate, land use and discharge efficiency. |
+| #68 | fix: change financial params to 2018 dollars & 5% interest rate. Start using terrain multipliers (which now include the economic multiplier). |