Add usage instructions and example output to the README

Author: nicwortel

README.md

@@ -7,3 +7,27 @@ These sniffs will report security vulnerabilities in your PHP code.
 This project is inspired
 by [phpcs-security-audit](https://github.com/FloeDesignTechnologies/phpcs-security-audit),
 a similar but no longer maintained project.
+
+## Usage
+
+Run PHP_CodeSniffer with the `Security` standard:
+
+```sh
+vendor/bin/phpcs --standard=Security /path/to/code
+```
+
+Example output:
+
+```text
+FILE: /path/to/file.php
+-----------------------------------------------------------------------------------------------------------------------
+FOUND 6 ERRORS AFFECTING 5 LINES
+-----------------------------------------------------------------------------------------------------------------------
+  5 | ERROR | Passing user input to require_once() can lead to remote file inclusion (CWE-98)
+  9 | ERROR | Passing user input to shell_exec() can lead to OS command injection (CWE-78)
+ 11 | ERROR | Outputting unsanitized user input can lead to cross-site scripting (CWE-79)
+ 11 | ERROR | Passing user input to file_get_contents() can lead to path traversal attacks (CWE-22)
+ 13 | ERROR | Cleartext transmission of data over HTTP is a security risk (CWE-319)
+ 15 | ERROR | The sha1 algorithm is not safe for password hashing (CWE-916), use password_hash() for hashing passwords
+-----------------------------------------------------------------------------------------------------------------------
+```