[Cassandra Wiki] Update of "HowToContribute" by Jim Witschey

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The "HowToContribute" page has been changed by Jim Witschey:
https://wiki.apache.org/cassandra/HowToContribute?action=diff&rev1=67&rev2=68

Comment:
documents use of known_failure decorator

    * `git clone https://github.com/riptano/cassandra-dtest.git cassandra-dtest`.
   1. Set `$CASSANDRA_DIR` to the location of your cassandra checkout.  For example: `export CASSANDRA_DIR=/home/joe/cassandra`.  Make sure you've already built Cassandra in this directory. You can build Cassandra by running `ant`.
   1. Run all tests by running `nosetests` from the dtest checkout.  You can run a specific module like so: `nosetests cql_tests.py`.  You can run a specific test method like this: `nosetests cql_tests.py:TestCQL.counters_test`.
+   * If you encounter any failures, you can confirm whether or not they exist in upstream branches by checking to see if the failing tests or test classes are tagged with the `known_failure` decorator. This decorator is [[https://github.com/riptano/cassandra-dtest/blob/master/tools.py#L329|documented inline in the dtests]]. If a test that is known to fail passes, or a test that is not known to fail succeeds, you should check the linked JIRA ticket to see if you've introduced any detrimental changes to that branch.
  
  === Running the code coverage task ===
   1. Run a basic coverage report of unit tests using `ant codecoverage`.