Update CHANGELOG.md

CHANGELOG.md

+1.2.3 - 01.31.20
+================
+* Update get-system git subcommand with a `--sha` optional parameter, along with tests and doc.
+* Add a basic `Makefile` (use `make check` to run tests).
+* Add script `merge_dev_into_master.sh`. It pulls the lastest changes, merges `development` on `master`, creates a tag and pushes.
+
 1.2.2 - 08.14.19
 ================
 * Add a script example to demonstrate data download, execution of the CLI, and validation of the outputs.