Add a SUPPORT.md file to the .github/ folder

[coliff]

It'd be good to add a SUPPORT file to the .github/ folder directing folks to Stack Overflow if they have a general question about using Bootstrap. A link to this file will display when someone opens a new issue which could cut down on new issues being opened unnecessarily.

It's a new GitHub feature - see:
https://github.com/blog/2400-support-file-support
https://help.github.com/articles/adding-support-resources-to-your-project/

[bardiharborow]

I'm not sure this adds much value compared to our pre-existing CONTRIBUTING.md and ISSUE_TEMPLATE.md (which no-one seems to read anyway), but I'll give it some thought.