Ruby: add file-level qldoc

Author: nickrolfe

ruby/ql/lib/codeql/ruby/regexp/RegExpInjectionCustomizations.qll

@@ -1,3 +1,8 @@
+/**
+ * Provides default sources, sinks and sanitizers for reasoning about regexp
+ * injection vulnerabilities, as well as extension points for adding your own.
+ */
+
 private import ruby
 private import codeql.ruby.DataFlow
 private import codeql.ruby.Concepts