Ignore uneeded files for distribution

[pH-7]

Change Summary

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 flag is specified).

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

In case 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 @kishorenc / @AbdullahFaqeir 😊

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