Add minimal Readme and contributing docs

Author: jrfnl

.github/CONTRIBUTING.md

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+Hi, thank you for your interest in contributing to the PHPCompatibility website! We look forward to working with you.
+
+Aside from a few specific pages, the documentation on the website is automatically generated from the source code of the [PHPCompatibility repository](https://github.com/PHPCompatibility/PHPCompatibility).
+
+The contents maintained in this repository are:
+- The pages in the "Using PHPCompatibility" section.
+- Site layout and structure.
+- CSS customizations.
+
+Pull requests to improve either of these are welcome.
+
+All the sniff and error code related content is auto-generated from a combination of:
+- The sniff class docblocks for each sniff in the [PHPCompatibility repo](https://github.com/PHPCompatibility/PHPCompatibility/PHPCompatibility/Sniffs)
+- The sniff specific additional documentation (if available) in the [PHPCompatibility repo](https://github.com/PHPCompatibility/PHPCompatibility/PHPCompatibility/Docs)
+
+Improvements to the sniff and error code related content are very welcome, but should be pulled to the [PHPCompatibility repository](https://github.com/PHPCompatibility/PHPCompatibility).
+
+
+## How the website is generated
+
+This website is a [GitHub Pages](https://docs.github.com/en/github/working-with-github-pages) site using the [Jekyll Site Generator](https://jekyllrb.com/) with the [Liquid template language](https://shopify.github.io/liquid/) with some [Jekyll specific additions to Liquid](https://jekyllrb.com/docs/liquid/).
+
+The website uses the [jekyll-rtd-theme](https://jekyll-rtd-theme.rundocs.io/) using the [rundocs core](https://rundocs.io/) with some site-specific customizations.
+
+The website is automatically updated whenever a commit is pushed to this repository.
+
+[TODO] Whenever a new release of the PHPCompatibility package is tagged, the documentation which is based on the source code of PHPCompatibility is automatically re-generated using the [PHPCS-docs](https://github.com/PHPCSStandards/phpcs-docs) generator package and committed to this repository via a GitHub actions workflow.
+
+
+## Testing the website locally
+
+* Install a full [Ruby development environment](https://jekyllrb.com/docs/installation/)
+* Clone this repository
+* Go to the root directory of the clone of this repository
+* Run `bundle update`
+* Run `bundle exec jekyll serve`
+* You should now be able to open the website in your preferred browser at: http://localhost:4000/
+
+For more information, see the [documentation on testing a GitHub Pages site locally](https://docs.github.com/en/github/working-with-github-pages/testing-your-github-pages-site-locally-with-jekyll).
+
+
+### Typical issues you may run into when testing the site locally
+
+#### The GitHub API credentials you provided aren't valid.
+
+Follow [these instructions](https://github.com/jekyll/github-metadata/blob/master/docs/authentication.md) on getting an access token set up.
+
+
+

README.md

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+PHPCompatibility
+----
+
+The source of the phpcompatibility.com website.
+