[java] Add DoNotExtendJavaLangThrowable rule

Copy and pasted code from DoNotExtendJavaLangError rule.

Resolves #367
https://github.com/pmd/pmd/issues/367

docs/pages/pmd/rules/java.md

@@ -326,6 +326,7 @@ List of rulesets and rules contained in each ruleset.
 *   [AvoidThrowingNullPointerException](pmd_rules_java_strictexception.html#avoidthrowingnullpointerexception): Avoid throwing NullPointerExceptions. These are confusing because most people will assume that th...
 *   [AvoidThrowingRawExceptionTypes](pmd_rules_java_strictexception.html#avoidthrowingrawexceptiontypes): Avoid throwing certain exception types. Rather than throw a raw RuntimeException, Throwable,Excep...
 *   [DoNotExtendJavaLangError](pmd_rules_java_strictexception.html#donotextendjavalangerror): Errors are system exceptions. Do not extend them.
+*   [DoNotExtendJavaLangThrowable](pmd_rules_java_strictexception.html#donotextendjavalangthrowable): Extend Exception or RuntimeException instead of Throwable.
 *   [DoNotThrowExceptionInFinally](pmd_rules_java_strictexception.html#donotthrowexceptioninfinally): Throwing exceptions within a 'finally' block is confusing since they may mask other exceptions or...
 *   [ExceptionAsFlowControl](pmd_rules_java_strictexception.html#exceptionasflowcontrol): Using Exceptions as form of flow control is not recommended as they obscure true exceptions when ...
 *   [SignatureDeclareThrowsException](pmd_rules_java_strictexception.html#signaturedeclarethrowsexception): Methods that declare the generic Exception as a possible throwable are not very helpful since the...

docs/pages/pmd/rules/java/strictexception.md

@@ -5,7 +5,7 @@ permalink: pmd_rules_java_strictexception.html
 folder: pmd/rules/java
 sidebaractiveurl: /pmd_rules_java.html
 editmepath: ../pmd-java/src/main/resources/rulesets/java/strictexception.xml
-keywords: Strict Exceptions, AvoidCatchingThrowable, SignatureDeclareThrowsException, ExceptionAsFlowControl, AvoidCatchingNPE, AvoidThrowingRawExceptionTypes, AvoidThrowingNullPointerException, AvoidRethrowingException, DoNotExtendJavaLangError, DoNotThrowExceptionInFinally, AvoidThrowingNewInstanceOfSameException, AvoidCatchingGenericException, AvoidLosingExceptionInformation
+keywords: Strict Exceptions, AvoidCatchingThrowable, SignatureDeclareThrowsException, ExceptionAsFlowControl, AvoidCatchingNPE, AvoidThrowingRawExceptionTypes, AvoidThrowingNullPointerException, AvoidRethrowingException, DoNotExtendJavaLangError, DoNotExtendJavaLangThrowable, DoNotThrowExceptionInFinally, AvoidThrowingNewInstanceOfSameException, AvoidCatchingGenericException, AvoidLosingExceptionInformation
 ---
 ## AvoidCatchingGenericException
 
@@ -310,6 +310,30 @@ public class Foo extends Error { }
 <rule ref="rulesets/java/strictexception.xml/DoNotExtendJavaLangError" />
 ```
 
+## DoNotExtendJavaLangThrowable
+
+**Since:** PMD 6.0
+
+**Priority:** Medium (3)
+
+Extend Exception or RuntimeException instead of Throwable.
+
+```
+//ClassOrInterfaceDeclaration/ExtendsList/ClassOrInterfaceType
+  [@Image="Throwable" or @Image="java.lang.Throwable"]
+```
+
+**Example(s):**
+
+``` java
+public class Foo extends Throwable { }
+```
+
+**Use this rule by referencing it:**
+``` xml
+<rule ref="rulesets/java/strictexception.xml/DoNotExtendJavaLangThrowable" />
+```
+
 ## DoNotThrowExceptionInFinally
 
 **Since:** PMD 4.2