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

Created by: 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/