added changelog

CHANGELOG.md

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+Release 1.0.5
+=============
+
+* In progress
+* Support multiple TOML configuration files #19
+
+Release 1.0.4
+=============
+
+* Released on July 1st, 2020
+* Support for processing Covariance matrices and hitmaps
+* Used to generate Covariance matrices and hitmaps for [`202006_reference_design`](https://github.com/CMB-S4/s4mapbasedsims/tree/master/202006_reference_design)
+
+Release 1.0.3
+=============
+
+* Released on June 16th, 2020
+* Used [`202006_reference_design`](https://github.com/CMB-S4/s4mapbasedsims/tree/master/202006_reference_design)