## What changes were proposed in this pull request?
Link to contributing wiki in PR template, README.md
## How was this patch tested?
Doc-only change, tested by Jekyll
Author: Sean Owen <sowen@cloudera.com>
Closes#15429 from srowen/SPARK-17840.
## What changes were proposed in this pull request?
(Please fill in changes proposed in this fix)
## How was the this patch tested?
(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)
Author: Rahul Tanwani <tanwanirahul@gmail.com>
Closes#11343 from tanwanirahul/pull_request_template.
