[doc] Mention rule categories

docs/pages/pmd/userdocs/understanding_rulesets.md

@@ -1,18 +1,43 @@
 ---
 title: Understanding Rulesets
 permalink: pmd_userdocs_understanding_rulesets.html
-summary: Rulesets are collections of rules
+summary: Rules belong to categories and rulesets are custom collections of rules
-last_updated: September 2017
+last_updated: November 2017
 ---
 
+## Rule Categories
+
+Since PMD 6.0.0, all provided built-in rules are sorted into one of eight categories:
+
+1.  **Best Practices**: These are rules which enforce generally accepted best practices.
+2.  **Code Style**: These rules enforce a specific coding style.
+3.  **Design**: Rules that help you discover design issues.
+4.  **Documentation**: These rules are related to code documentation.
+5.  **Error Prone**: Rules to detect constructs that are either broken, extremely confusing or prone to runtime errors.
+6.  **Multithreading**: These are rules that flag issues when dealing with multiple threads of execution.
+7.  **Performance**: Rules that flag suboptimal code.
+8.  **Security**: Rules that flag potential security flaws.
+
+These categories help you to find rules and figure out the relevance and impact for your project.
+
+You can find the available rules under "Rule Reference" in the menu. Each languages has its own rule
+index, e.g. [Java Rules](pmd_rules_java.html) or [JavaScript Rules](pmd_rules_ecmascript.html).
+
+{% include note.html content="Not all supported languages provide rules in all categories yet. " %}
+
+
+## Rulesets
+
 There are two major use cases:
 
 1.  When defining a new rule, the rule needs to be defined in a ruleset. PMD's built-in rules
-    are defined in special rulesets from which the rule reference documentation is generated,
+    are defined in special rulesets which form the eight categories mentioned above.
+    From these rulesets the rule reference documentation is generated,
     see [Java Rules](pmd_rules_java.html) for an example.
 
     Similar rules are grouped together into the same category, like [Java Best Practices](pmd_rules_java_bestpractices.html)
-    which contains rules which enforce generally accepted best practices.
+    which contains rules which enforce generally accepted best practices. Each category uses its own
+    ruleset file.
 
 2.  When executing PMD, you need to tell, which rules should be executed. You could directly point to the
     built-in rulesets, but then you might be overwhelmed by the found violations. As described