update changelog with release date

Author: isaisabel

CHANGELOG.md

@@ -1,14 +1,11 @@
-# Changes staged on develop
+# v1.5 - 8 July 2020
 ## New Scripts
 Added scripts used to generate the [sample layers in the ATT&CK Navigator repository](https://github.com/mitre-attack/attack-navigator/tree/develop/layers/data/samples). See issue [#21](https://github.com/mitre-attack/attack-scripts/issues/21) and [the sample layer README](scripts/layers/samples/README.md) for more details. The following scripts were added:
 - [heatmap.py](scripts/layers/samples/heatmap.py)
 - [bear_APT.py](scripts/layers/samples/bear_APT.py)
 - [apt3_apt29_software.py](scripts/layers/samples/apt3_apt29_software.py)
 - [software_execution.py](scripts/layers/samples/software_execution.py)
 
-## Improvements
-- Updated [diff_stix.py](scripts/diff_stix.py) with sub-techniques support. See issue [#12](https://github.com/mitre-attack/attack-scripts/issues/12).
-
 # V1.3 - 8 January 2019
 ## New Scripts
 - Added [diff_stix.py](scripts/diff_stix.py).