Added documentation

docs/en/plugins/shell.md

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+Runs a given Shell command.
+
+**Note: ** Because this plugin could potentially be abused, it requires extra steps to enable it:
+
+1. In the root of your PHPCI system, in the same directory where you'll find composer.json and vars.php, look for a file local_vars.php. If it does not exist, create it.
+2. In local_vars.php add this code:
+
+```php
+<?php
+
+define('ENABLE_SHELL_PLUGIN', true);
+```
+
+If `ENABLE_SHELL_PLUGIN` is either false or undefined, the shell plugin won't work.
+
+### Configuration Options:
+
+* **command** - Required - The shell command to run.
+
+```yml
+setup:
+    shell:
+        command: "bin/console build"
+```
+ You should understand, that in old configuration type, you can run only one command!
+
+### New format of Configuration Options
+
+```yml
+setup:
+   shell:
+       - "cd /www"
+       - "chmod u+x %BUILD_PATH%/bin/console"
+       - "%BUILD_PATH%/bin/console build"
+```
+
+#### Each new command forgets about what was before
+
+So if you want cd to directory and then run script there, combine those two commands into one like:
+
+```yml
+setup:
+    shell:
+        - "cd %BUILD_PATH% && php artisan migrate" # Laravel Migrations
+```
+
+
+[See variables which you can use in shell commands](../interpolation.md)