[SPARK-6889] [DOCS] CONTRIBUTING.md updates to accompany contribution doc updates #5623
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Part of the SPARK-6889 doc updates, to accompany wiki updates at https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark
See draft text at https://docs.google.com/document/d/1tB9-f9lmxhC32QlOo4E8Z7eGDwHx1_Q3O8uCmRXQTo8/edit#