ARROW-11852: [Docs] Update CONTRIBUTING to explain Contributor role
ARROW-11852: [Docs] Update CONTRIBUTING to explain Contributor role

Closes #9648 from dianaclarke/ARROW-11852

Authored-by: Diana Clarke <22b83d3550a2682d05bc07a63e9a9fa5dd411e0a@gmail.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

wesm/arrow,icexelloss/arrow,laurentgo/arrow,wesm/arrow,kou/arrow,wesm/arrow,kou/arrow,laurentgo/arrow,icexelloss/arrow,wesm/arrow,laurentgo/arrow,icexelloss/arrow,apache/arrow,laurentgo/arrow,kou/arrow,icexelloss/arrow,apache/arrow,laurentgo/arrow,kou/arrow,kou/arrow,cpcloud/arrow,kou/arrow,kou/arrow,icexelloss/arrow,kou/arrow,icexelloss/arrow,wesm/arrow,icexelloss/arrow,wesm/arrow,cpcloud/arrow,wesm/arrow,wesm/arrow,kou/arrow,apache/arrow,laurentgo/arrow,icexelloss/arrow,cpcloud/arrow,icexelloss/arrow,kou/arrow,wesm/arrow,wesm/arrow,wesm/arrow,apache/arrow,wesm/arrow,apache/arrow,cpcloud/arrow,kou/arrow,icexelloss/arrow,cpcloud/arrow,cpcloud/arrow,kou/arrow,kou/arrow,apache/arrow,laurentgo/arrow,laurentgo/arrow,cpcloud/arrow,laurentgo/arrow,icexelloss/arrow,laurentgo/arrow,cpcloud/arrow,apache/arrow,laurentgo/arrow,cpcloud/arrow,apache/arrow,apache/arrow,apache/arrow,apache/arrow,laurentgo/arrow,icexelloss/arrow,kou/arrow,apache/arrow,laurentgo/arrow,cpcloud/arrow,apache/arrow,apache/arrow,laurentgo/arrow,icexelloss/arrow,wesm/arrow,laurentgo/arrow,cpcloud/arrow,cpcloud/arrow,cpcloud/arrow,wesm/arrow,icexelloss/arrow,cpcloud/arrow