Add README.md with rule description and examples of work

README.md

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+PHP Line Length Checker
+=======================
+
+Custom rule for [PHP-CS-Fixer](https://github.com/PHP-CS-Fixer/PHP-CS-Fixer).  
+Checks that the length of lines does not exceed 120 characters.  
+Doesn't fix anything.  
+
+### How it works
+Searches for the line that exceed max length limit and mark it by adding comment:  
+``` # Line too long```  
+
+To see which lines violates the rule you should run the command with the`--diff`  flag.  
+
+DIff example:
+<p>
+    <img src="./diff-example.png" title="Diff Example" alt="Diff Example">
+</p>
+
+Works only for `check` command and `fix` command with `--dry-run` flag. Does nothing in other cases.
+
+
+### Installation
+Install package:
+```console
+composer require --dev gordinskiy/line-length-checker
+```
+Register rule in PHP-CS-Fixer config file:
+```php
+<?php
+// ...
+return (new PhpCsFixer\Config())
+    // ...
+    ->registerCustomFixers([
+        new Gordinskiy\LineLengthChecker\Rules\LineLengthLimit()
+    ])
+;
+```
+
+Enable rule in PHP-CS-Fixer config file:
+```php
+<?php
+// ...
+return (new PhpCsFixer\Config())
+    // ...
+    ->setRules([
+        'Gordinskiy/line_length_limit' => true,
+    ])
+;
+```
+
+### Configuration
+Default limitation is 120, but it can be configured by `max_length` option:
+```php
+<?php
+// ...
+return (new PhpCsFixer\Config())
+    // ...
+    ->setRules([
+        'Gordinskiy/line_length_limit' => ['max_length' => 115],
+    ])
+;
+```