feat: add Git Attributes (#191)

This adds a `.gitattributes` file to exclude tests and other unnecessary files from production releases.

Author: owenvoke

.gitattributes

@@ -0,0 +1,14 @@
+* text=auto
+
+# Path-based git attributes
+# https://git-scm.com/docs/gitattributes
+
+# Ignore all test and documentation with "export-ignore".
+/.editorconfig      export-ignore
+/.gitattributes     export-ignore
+/.github            export-ignore
+/.gitignore         export-ignore
+/.styleci.yml       export-ignore
+/CODE_OF_CONDUCT.md export-ignore
+/phpunit.xml        export-ignore
+/tests              export-ignore