Initial commit / v 1.0.0

[jrfnl]

• Adds the WordPress specific PHPCompatibility ruleset.
• Adds README with basic installation and usage instructions.
• Adds composer.json file which requires the PHPCompatibility standard.
• Adds a Travis config to validate the ruleset XML and the Composer configuration on each update.
• Adds .gitignore to ignore the vendor directory and the composer.lock file.
• Adds .gitattributes to keep release archives clean.
• Adds .github/issue_template.md to try and prevent issues related to the generic PHPCompatibility standard from being opened in this repository.

FYI: The build won't pass until PR https://github.com/PHPCompatibility/PHPCompatibility/pull/689 has been merged and the package has been updated on Packagist.

[jrfnl]

For future reference - this PR is based on and supersedes the following PRs which were previously merged in the PHPCompatibility repo:

• https://github.com/PHPCompatibility/PHPCompatibility/pull/615
• https://github.com/PHPCompatibility/PHPCompatibility/pull/648

Issue which initiated the framework/CMS specific rulesets: https://github.com/PHPCompatibility/PHPCompatibility/issues/530