Create CHANGELOG.md

Author: CarsonDavis

CHANGELOG.md

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+## Overview
+These are not the release notes, which can be found https://github.com/NASA-IMPACT/COSMOS/releases. Instead, this is a changelog that developers use to log key changes to the codebase with each pull request.
+
+## What to Include
+For each PR made, an entry should be added to this changelog. It should contain 
+- a brief description of the deliverable of the feature or bugfix
+- exact listing of key changes such as:
+  - API endpoint modified
+  - frontend components added
+  - model updates
+  - deployment changes needed on the servers
+  - etc.
+
+## Changelog