General: Work item PHPOfficeGH-98 - Split repository in order to improve distribution of the library via composer

Added .gitattributes file to identify files/folders that should be excluded from a composer distribution unless invoked as
    composer install --prefer-source

Author: Mark Baker

.gitattributes

@@ -0,0 +1,4 @@
+/Build export-ignore
+/Documentation export-ignore
+/Tests export-ignore
+README.md export-ignore