[Hadoop Wiki] Update of "HowToContribute" by JunpingDu

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The "HowToContribute" page has been changed by JunpingDu:
https://wiki.apache.org/hadoop/HowToContribute?action=diff&rev1=88&rev2=89

    * You must implement a class that uses {{{@Test}}} annotations for all test methods. Please note, [[http://wiki.apache.org/hadoop/HowToDevelopUnitTests|Hadoop uses JUnit v4]].
    * Define methods within your class whose names begin with {{{test}}}, and call JUnit's many assert methods to verify conditions; these methods will be executed when you run {{{mvn test}}}. Please add meaningful messages to the assert statement to facilitate diagnostics.
    * By default, do not let tests write any temporary files to {{{/tmp}}}.  Instead, the tests should write to the location specified by the {{{test.build.data}}} system property.
-   * If a HDFS cluster or a MapReduce cluster is needed by your test, please use {{{org.apache.hadoop.dfs.MiniDFSCluster}}} and {{{org.apache.hadoop.mapred.MiniMRCluster}}}, respectively.  {{{TestMiniMRLocalFS}}} is an example of a test that uses {{{MiniMRCluster}}}.
+   * If a HDFS cluster or a MapReduce/YARN cluster is needed by your test, please use {{{org.apache.hadoop.dfs.MiniDFSCluster}}} and {{{org.apache.hadoop.mapred.MiniMRCluster}}} (or {{{org.apache.hadoop.yarn.server.MiniYARNCluster}}}), respectively.  {{{TestMiniMRLocalFS}}} is an example of a test that uses {{{MiniMRCluster}}}.
    * Place your class in the {{{src/test}}} tree.
    * {{{TestFileSystem.java}}} and {{{TestMapRed.java}}} are examples of standalone MapReduce-based tests.
    * {{{TestPath.java}}} is an example of a non MapReduce-based test.