Add .gitattributes to syntax highlight bst files as yaml

Add a `.gitattributes` file to suggest GitHub Linguist to treat
`.bst` files and `project.conf` file anywhere in the project as
YAML for the purposes of syntax highlighting.

If we agree with this change, we can also add something in our
documentation to suggest users to add something similar to
their projects. Although that suggestion will depend on the Git
hosting service used. GitHub and GitLab both support overrides
via `.gitattributes` file but use different keywords. See:

* https://github.com/github-linguist/linguist/blob/master/docs/overrides.md#using-gitattributes
* https://docs.gitlab.com/ee/user/project/highlighting.html#override-syntax-highlighting-for-a-file-type
1 file changed