[TASK] Move the contribution guidelines to the project root (#699)

This hopefully makes them a bit easier to find.

Fixes #676

Author: oliverklee

.gitattributes

@@ -4,6 +4,7 @@
 /.gitignore export-ignore
 /.phive/ export-ignore
 /CODE_OF_CONDUCT.md export-ignore
+/CONTRIBUTING.md export-ignore
 /bin/ export-ignore
 /config/ export-ignore
 /phpunit.xml export-ignore

.github/CONTRIBUTING.md renamed to CONTRIBUTING.md

@@ -784,7 +784,7 @@ classDiagram
 
 Contributions in the form of bug reports, feature requests, or pull requests are
 more than welcome. :pray: Please have a look at our
-[contribution guidelines](.github/CONTRIBUTING.md) to learn more about how to
+[contribution guidelines](CONTRIBUTING.md) to learn more about how to
 contribute to PHP-CSS-Parser.
 
 ## Contributors/Thanks to