Update Prow setup document (#1441)
Update Prow setup document (#1441)

* Update Prow setup document

* remove irrelevant sections
* update section about Boskos
* add section about setting up the issue tracker

* Fix script name/location in example

* Fix markdown

* Fix markdown (again)

* Update link to issue_tracker_config.go

* Update instructions

knative/test-infra,knative/test-infra,knative/test-infra