Add .gitattributes to slim down composer packages

When composer is used as a dependency manager for large web apps, transferring lots of extra files (such as tests and documentation) can be very time and bandwidth consuming. By using .gitattributes we can tell composer what files to ignore when distributing through composer.

.gitattributes

@@ -0,0 +1,8 @@
+/.editorconfig export-ignore
+/.gitattributes export-ignore
+/.gitignore export-ignore
+/.scrutinizer.yml export-ignore
+/.travis.yml export-ignore
+/README.md export-ignore
+/phpunit.xml.dist export-ignore
+/tests export-ignore