add .gitattributes export-ignore entries to reduce the release package size (#35)

Author: arcticlinux

.gitattributes

@@ -0,0 +1,14 @@
+# Ignore all test and documentation with "export-ignore".
+/.gitattributes     export-ignore
+/.github            export-ignore
+/.gitignore         export-ignore
+/.php.cs            export-ignore
+/.scrutinizer.yml   export-ignore
+/.travis.yml        export-ignore
+/CHANGELOG.md       export-ignore
+/CONTRIBUTING.md    export-ignore
+/README.md          export-ignore
+/build.xml          export-ignore
+/phpunit.xml        export-ignore
+/scientist.png      export-ignore
+/tests              export-ignore