CONTRIBUTING: move file to .github directory

... which is nowadays the more common location for this type of file.

Includes updating the link in the README file.

Ref: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

Author: jrfnl

CONTRIBUTING.md renamed to .github/CONTRIBUTING.md

@@ -32,7 +32,7 @@ Fixes #
 ## PR checklist
 <!-- Go over all the following points, and put an `x` in all the boxes that apply. -->
 - [ ] I have checked there is no other PR open for the same change.
-- [ ] I have read the [Contribution Guidelines](.github/CONTRIBUTING.md).
+- [ ] I have read the [Contribution Guidelines](https://github.com/PHPCSStandards/PHP_CodeSniffer/blob/master/.github/CONTRIBUTING.md).
 - [ ] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
 - [ ] I have added tests to cover my changes.
 - [ ] I have verified that the code complies with the projects coding standards.

.github/pull_request_template.md

@@ -99,7 +99,7 @@ Bug reports and feature requests can be submitted on the [Github Issue Tracker](
 
 ## Contributing
 
-See [CONTRIBUTING.md](CONTRIBUTING.md) for information.
+See [CONTRIBUTING.md](.github/CONTRIBUTING.md) for information.
 
 ## Versioning