Add .gitattributes to exclude unneeded files from dist zip archive

[pH-7]

Summary

A good idea is to exclude unneeded dev files from the final production zip archive file, which is downloaded by composer (unless --prefer -source is specified). Those files are only useful when working on the library itself.

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

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

Hope it helps

[crookedneighbor]

This seems reasonable to me. There are a few more files that are for dev only though:

.github/
Dockerfile
Makefile
Rakefile

I'd probably keep the CHANGELOG.md in. That and the README are sometimes valuable to inspect when pulling down a package.

[pH-7]

Sure, makes sense @crookedneighbor. I updated the list :)