Heroku requires the file `app.json` as well as the composer files
`composer.json` and `composer.lock` to deploy a service. Deploy
doesn't work if these files are ignored during export (because of
the way this service deploys projects).
This commit adds comments to .gitattributes to prevent this issue
from re-appearing in the future. All affected lines are commented
out.
Also added some spacing for better readability.
References #1165
Files with the option "export-ignore" are excluded from "git archive"
commands. Release files from GitHub will also ignore those files, so
packages are smaller and don't include unneccessary files.
