Added documentation

docs/en/plugins/php_code_sniffer.md

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+Runs PHP Code Sniffer against your build.
+
+## Configuration
+### Options
+* **allowed_warnings** [int, optional] - The warning limit for a successful build.
+* **allowed_errors** [int, optional] - The error limit for a successful build.
+* **suffixes** [array, optional] - An array of file extensions to check.
+* **standard** [string, optional] - The standard against which your files should be checked (defaults to PSR2.)
+* **tab_width** [int, optional] - Your chosen tab width.
+* **encoding** [string, optional] - The file encoding you wish to check for.
+* **path** [string, optional] - Path in which to run PHP Code Sniffer.
+* **ignore** [array, optional] - A list of files / paths to ignore, defaults to the build_settings ignore list.
+
+### Example
+Simple example where PHPCS will run on app directory, but ignore the views folder, and use PSR-1 and PSR-2 rules for validation:
+```yml
+test:
+    php_code_sniffer:
+        path: "app"
+        ignore:
+            - "app/views"
+        standard: "PSR1,PSR2"
+```
+
+For use with an existing project:
+```yml
+test:
+    php_code_sniffer:
+        standard: "/phpcs.xml" # The leading slash is needed to trigger an external ruleset.
+                               # Without it, PHPCI looks for a rule named "phpcs.xml"
+        allowed_errors: -1 # Even a single error will cause the build to fail. -1 = unlimited
+        allowed_warnings: -1
+```