Add export-ignore to .gitattributes

Removes unneeded files from releases.

Author: joedolson

.gitattributes

@@ -15,3 +15,20 @@
 *.PDF	 diff=astextplain
 *.rtf	 diff=astextplain
 *.RTF	 diff=astextplain
+
+.gitattributes export-ignore
+.gitignore export-ignore
+.github export-ignore
+.github/** export-ignore
+bin export-ignore
+bin/** export-ignore
+docs export-ignore
+docs/** export-ignore
+tests export-ignore
+tests/** export-ignore
+phpcs.xml.dist export-ignore
+phpunit.xml.dist export-ignore
+README.md export-ignore
+hookdoc-conf.json export-ignore
+package.json export-ignore
+composer.json export-ignore