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

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.