Add `.gitattributes` file to prevent user from downloading useless files

[Micael Levi L. Cavalcante]

Hey!

By now, when we click on GitHub .zip download hyperlink we download a bunch of files that we won't use in the end. Thus I think it would be nice to remove them when downloading by using the export-ignore attribute

before (81.1kB) ✖️	after (1.5kB) ✔️

You just need to add the following file in the project's root:

.gitattributes

## Path-based git attributes
## https://www.kernel.org/pub/software/scm/git/docs/gitattributes.html
## The following files won't be added to archived files:  
/backlinks.txt      export-ignore
/LICENSE            export-ignore
/.gitattributes     export-ignore
/.github            export-ignore 
/README.md          export-ignore 
/screenshot.png     export-ignore 

it's pretty simple so why not? 😄

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Inspired by this article by @joaorobertopb

[Aurélien Lourot]

Thanks @micalevisk, good idea, done :)