Update release notes markdown for 0.20.0

Author: mattstratton

RELEASE_NOTES.md

@@ -1,11 +1,11 @@
 # Change Log
 
-## [0.19.1](https://github.com/devopsdays/devopsdays-cli/tree/0.19.1) (2017-10-07)
-[Full Changelog](https://github.com/devopsdays/devopsdays-cli/compare/0.19.0...0.19.1)
+## [0.20.0](https://github.com/devopsdays/devopsdays-cli/tree/0.20.0) (2017-10-08)
+[Full Changelog](https://github.com/devopsdays/devopsdays-cli/compare/0.19.1...0.20.0)
 
 **Implemented enhancements:**
 
-- Check for png on event logo [\#108](https://github.com/devopsdays/devopsdays-cli/issues/108)
+- Create showSpeaker function [\#58](https://github.com/devopsdays/devopsdays-cli/issues/58)