Updated CHANGELOG to v0.9.12

CHANGELOG.md

 # Changelog
 
+### v0.9.12 <small>_November 29, 2019_</small>
+* CI: Replaced packr2 with pkger
+* Server: Fixed data race on broadcast set
+* Server: Moved Version and DefaultTwitchClientID to config.go in project root
+* Server: Formatted static assets
+* Repo: Added .prettierignore for markdown files because pymdown flavored markdown is currently not supported
+
 ### v0.9.11 <small>_November 3, 2019_</small>
 * CI: Using pymdown-env include $VERSION in docs
 * Docs: Fixed relative links