Button Component – Add Missing Files

Status: **Ready for Review**
Reviewers: @kpeatt @ry5n @avelinet @cole-sanderson @nastiatikk @mlworks 
JIRA: **CSOPS-842**

## Changes
- Added dust template with example markup
- Added readme.md
- Added javascript data file
- Fix linting error in _button.scss

## Notes
- Added a note to the button readme that highlights a work around discussed in PR #77 for the issue as described in #76