update changelog for st2 lesson
- Adding Lesson-30 Working with Salt [#114](https://github.com/nre-learning/antidote/pull/114)
- Adding Lesson-16 Jinja2 Templates [#121](https://github.com/nre-learning/antidote/pull/121)
+- Adding Lesson-15 Event-Driven Automation with StackStorm [#126](https://github.com/nre-learning/antidote/pull/126)
### Other
