changelog for v0.2.0

Author: jpoly1219

changelogs/changelog_v020.md

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+# Changelog for v0.2.0
+
+## New features:
+- Plotting is here! Use `Plot` to plot your DataFrame object.
+  - Plotting uses gnuplot as a backend. To use the plotting feature, you need to install gnuplot first.
+  - This feature is quite basic at the moment; more features and granular control will be added in later updates!