Ignore uneeded files for distribution

A good idea is to exclude unneeded dev files from the final production zip archive file, which is also downloaded by composer (unless --prefer -source is specified).

Those files are actually only useful when working on the library itself.

If you are not familiar, further information about `export-ignore` can be found below for instance:

* http://www.pixelite.co.nz/article/using-git-attributes-exclude-files-your-release/
* https://madewithlove.be/gitattributes/

I hope it helps! 😊

Author: pH-7

.gitattributes

@@ -0,0 +1,4 @@
+/examples       export-ignore
+.gitattributes  export-ignore
+.gitignore      export-ignore
+phpcs.xml       export-ignore