Ignore dev/testing files for distribution builds

[pH-7]

.gitattributes file was missing from the client-php. It's actually a good idea to exclude unneeded dev files from the the production archive, which is used by composer for instance.

The files I mentioned are only useful when working on the library itself.

If you are not familiar with .gitattributes/export-ignore, you can find further info below:

• http://www.pixelite.co.nz/article/using-git-attributes-exclude-files-your-release/

I hope it helps! 🙂

[pH-7]

@estevanjantsk Could you have a look? 🤗