Add what PHPMD does

code-scanning/phpmd.yml

@@ -2,6 +2,13 @@
 # They are provided by a third-party and are governed by
 # separate terms of service, privacy policy, and support
 # documentation.
+# What PHPMD does is: It takes a given PHP source code base 
+# and look for several potential problems within that source.
+# These problems can be things like:
+#   Possible bugs
+#   Suboptimal code
+#   Overcomplicated expressions
+#   Unused parameters, methods, properties
 # PHPMD is a spin-off project of PHP Depend and 
 # aims to be a PHP equivalent of the well known Java tool PMD.
 # More details at https://phpmd.org/