📚 Add Readme with installation and usage instructions

Add badges, installation instructions, information on the ruleset and how to use it, a contributing section and license information to the Readme.

Author: jrfnl

README.md

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+PHPCSDevCS for developers of PHP_CodeSniffer sniffs
+=====================================================
+
+[![Latest Stable Version](https://poser.pugx.org/phpcsstandards/phpcsdevcs/v/stable)](https://packagist.org/packages/phpcsstandards/phpcsdevcs)
+[![Release Date of the Latest Version](https://img.shields.io/github/release-date/PHPCSStandards/PHPCSDevCS.svg?maxAge=1800)](https://github.com/PHPCSStandards/PHPCSDevCS/releases)
+:construction:
+[![Latest Unstable Version](https://img.shields.io/badge/unstable-dev--master-e68718.svg?maxAge=2419200)](https://packagist.org/packages/phpcsstandards/phpcsdevcs#dev-master)
+[![Travis Build Status](https://travis-ci.com/PHPCSStandards/PHPCSDevCS.svg?branch=master)](https://travis-ci.com/PHPCSStandards/PHPCSDevCS)
+[![Last Commit to Unstable](https://img.shields.io/github/last-commit/PHPCSStandards/PHPCSDevCS/master.svg)](https://github.com/PHPCSStandards/PHPCSDevCS/commits/master)
+
+[![Minimum PHP Version](https://img.shields.io/packagist/php-v/phpcsstandards/phpcsdevcs.svg?maxAge=3600)](https://packagist.org/packages/phpcsstandards/phpcsdevcs)
+[![License: LGPLv3](https://poser.pugx.org/phpcsstandards/phpcsdevcs/license)](https://github.com/PHPCSStandards/PHPCSDevCS/blob/master/LICENSE)
+![Awesome](https://img.shields.io/badge/awesome%3F-yes!-brightgreen.svg)
+
+
+This is an external ruleset for [PHP CodeSniffer](https://github.com/squizlabs/PHP_CodeSniffer) intended for use by sniff developers.
+
+* [Installation](#installation)
+    + [Composer Project-based Installation](#composer-project-based-installation)
+    + [Composer Global Installation](#composer-global-installation)
+* [PHPCSDev ruleset for sniff repos](#phpcsdev-ruleset-for-sniff-repos)
+    + [Using the ruleset from the command line](#using-the-ruleset-from-the-command-line)
+    + [Using the ruleset from within a project based ruleset](#using-the-ruleset-from-within-a-project-based-ruleset)
+* [Contributing](#contributing)
+* [License](#license)
+
+
+Installation
+-------------------------------------------
+
+### Composer Project-based Installation
+
+Run the following from the root of your project:
+```bash
+composer require --dev phpcsstandards/phpcsdevcs:"^1.0"
+```
+
+### Composer Global Installation
+
+If you work on several different sniff repos, you may want to install this toolset globally:
+```bash
+composer global require --dev phpcsstandards/phpcsdevcs:"^1.0"
+```
+
+Composer will automatically install dependencies and register the PHPCSDev and other external standards with PHP_CodeSniffer using the [DealerDirect Composer PHPCS plugin](https://github.com/Dealerdirect/phpcodesniffer-composer-installer/).
+
+
+PHPCSDev ruleset for sniff repos
+------------------------------
+
+Once this project is installed, you will see a new `PHPCSDev` ruleset in the list of installed standards when you run `phpcs -i`.
+
+**Important: This ruleset currently requires PHP_CodeSniffer >= `3.5.0+`.**
+
+> As sniffs developers will mostly work with the latest version of PHP_CodeSniffer, this shouldn't cause any problems.
+>
+> Similarly, the CS check in automated CI runs should normally be run on a high PHPCS version for the best results.
+
+The `PHPCSDev` standard can be used by sniff developers to check the code style of their sniff repo code.
+
+Often, sniff repos will use the code style of the standard they are adding. However, not all sniff repos are actually about code style.
+
+So for those repos which need a basic standard which will still keep their code-base consistent, this standard should be useful.
+
+The standard checks your code against the following:
+* Compliance with [PSR-12](https://www.php-fig.org/psr/psr-12/), with a few exceptions.
+* Use of camelCase variable and function names.
+* Use of normalized arrays.
+* All files, classes, functions and properties are documented with a docblock and contain the minimally needed information.
+* A number of arbitrary additional code style and QA checks.
+* PHP cross-version compatibility, while allowing for the tokens back-filled by PHPCS itself.
+    Note: for optimal results, the project custom ruleset should set the `testVersion` config variable.
+    This is not done by default as config variables are currently [difficult](https://github.com/squizlabs/PHP_CodeSniffer/issues/2197) [to overrule](https://github.com/squizlabs/PHP_CodeSniffer/issues/1821).
+
+The ruleset can be used like any other ruleset and specific sniffs and settings can be added to or overruled from a custom project based ruleset.
+
+### Using the ruleset from the command line
+```bash
+phpcs . --standard=PHPCSDev
+```
+
+### Using the ruleset from within a project based ruleset
+
+Add the following line to your project's `phpcs.xml.dist` file:
+```xml
+<rule ref="PHPCSDev"/>
+```
+
+
+Contributing
+-------
+Contributions to this project are welcome. Just clone the repo, branch off from `master`, make your changes, commit them and send in a pull request.
+
+If unsure whether the changes you are proposing would be welcome, open an issue first to discuss your proposal.
+
+License
+-------
+This code is released under the GNU Lesser General Public License (LGPLv3). For more information, visit http://www.gnu.org/copyleft/lesser.html