[Cassandra Wiki] Update of "HowToContribute" by RussellHatch

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The "HowToContribute" page has been changed by RussellHatch:
https://wiki.apache.org/cassandra/HowToContribute?action=diff&rev1=59&rev2=60

Comment:
update code coverage info from cobertura to jacoco

   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`
  
  === Running the code coverage task ===
-  1. Unzip this one: http://sourceforge.net/projects/cobertura/files/cobertura/1.9.4.1/cobertura-1.9.4.1-bin.zip/download
-  1. `ant codecoverage -Dcobertura.dir=/path/to/cobertura`
-  1. `/path/to/cobertura/cobertura-report.sh --destination build/cobertura/html source code src/java`
-  1. View `build/cobertura/html/index.html`
+  1. Run a basic coverage report of unit tests using `ant codecoverage`
+  1. Alternatively, run any test task with `ant jacoco-run -Dtaskname=some_test_taskname`. Run more test tasks in this fashion to push more coverage data onto the report in progress. Then manually build the report with `ant jacoco-report` (the 'codecoverage' task shown above does this automatically).
+  1. View the report at `build/jacoco/index.html`.
+  1. When done, clean up jacoco data so it doesn't confuse your next coverage report: `ant jacoco-cleanup`.
  
  === Continuous integration ===
  Jenkins runs the Cassandra tests continuously: http://cassci.datastax.com/ (Builders for stable branches also exist.)