Add `.gitattributes` file to exclude files not needed in composer installs.

[peterwilsoncc]

The following files can be excluded from composer installs:

• phpunit config
• phpcs config
• travis config
• docs directory
• tests directory

It sure would be dandy if these were listed in a .gitattributes file with the export-ignore flag to avoid putting test code on public servers.

Note: this applies to other HM projects too but I'm not going to create dozens of issues because that would be more than a little passive aggressive.

[rmccue]

Good idea 👍 We could potentially use archive.ignore in composer.json instead, but I think the gitattributes probably makes more sense.