.gitattributes files are a useful way to mark certain metadata about paths in a git repository (https://git-scm.com/docs/gitattributes). This can include all kinds of information, but one of the more useful ones is that linguist (https://github.com/github/linguist, the tool that powers the language statistics/diff view of GitHub) respects certain attributes. For example:

* `linguist-vendored` paths will not be counted toward the language statistics
* `linguist-generated` paths will not have their diff shown by default

These little niceties can be really helpful over time, so we should add it to the default app generator.